Materials: a flat board for the ramp (a cutting board, the hard cover of a picture book, or a flat piece of cardboard all work), propped up at one end with a few books. Then gather a pile of things that can roll: toy cars, a ball, a soda can, an orange, a rolled-up tube of paper. The game: place objects at the top of the ramp, let go at the same time, and see who reaches the bottom first. Then change the slope (add more books to make it steeper) and race again; swap in different weights and shapes and see what changes. Before each race, have your child guess out loud: which one's faster?
She's in charge of "let go and roll"—set the ball at the top, wait for "ready... go!", let go, and watch it tumble down. That "let go and it moves" cause-and-effect is irresistible to her; she'll do it dozens of times without getting bored. Let her pick "which ball today," then chase the runaway ball and bring it back—fetching the ball is a great big-muscle game all on its own.
She runs a "controlled experiment"—change just one thing at a time: first fix the slope and compare different objects; then keep the same ball and compare different slopes. Have her predict and explain: why do round things roll but square things don't? Why does a steeper ramp mean faster? (When it's steeper, more of gravity's downward pull goes into driving it forward.) She can use a ruler and "how many seconds" to time each run and compare.
Big sister is "referee + scorekeeper"—she calls the start, judges who reaches the bottom first, and writes down results; little sister is "launcher + ball-fetcher"—she lets go to start the roll and retrieves anything that rolls away (no overlap: judging and recording go to big sister, launching and running go to little sister). Hold a "rolling tournament," rank the different balls, and then build one longer, curvier track together so the ball has to turn a corner before the finish.
An object on a ramp is being "pulled down" by Earth's gravity; the steeper the slope, the more of that pull turns into forward motion, so it goes faster. A round object turns that pull into rolling, while a square one just gets stuck—shape decides whether it rolls or stops. Twist: put an empty box at the bottom as a "goal" and see who can make the ball stop right inside it—great practice for a feel for force and distance.
Materials: a large basin or sink half-filled with water, plus a pile of small things from around the house—a spoon, a building block, an eraser, a bottle cap, an orange, a grape, a coin, a plastic toy, a ball of foil, a small chunk of carrot. The game: before each item goes in, everyone guesses first—float or sink?—then drop it in to check. Clap when you're right, and cheer even louder when you're wrong—surprises are the fun part. Sort the items into two piles, "floaters" and "sinkers," and look for what they have in common.
She's in charge of "plop it in and scoop it out"—drop things in one by one for the "plop," watch them bob up or sink down, then fish them back with a little strainer or her hands. The feel of the water and the sounds are the whole joy for her. Put words to it for her: "This one floats—on top! This one sinks—to the bottom!" Use body motions (hands up / hands pressing down) to help those two words stick.
She "finds the pattern and breaks the exceptions"—after sorting the two piles, look for what they share (heavier = sinks? do big things always sink?). Give her two challenges that defy intuition: a big block of wood floats while a tiny nail sinks, showing it's not about size or weight but "which is heavier for the same size" (density); then hand her a ball of foil—rolled solid it sinks, pinched into a boat it floats. Let her work out why (the boat traps air, so the whole thing becomes "lighter").
Big sister is "experiment host"—she names the next item, tallies everyone's guesses, and sorts the piles; little sister is "dropper + retriever"—she drops things in, fishes them out, and announces "float or sink" (no overlap: sorting and recording go to big sister, going in and fishing out go to little sister). Run a "foil-boat cargo race" together: pinch a foil boat, gently load coins (big sister places, little sister counts), and see how many it takes to sink—whose boat carries the most?
Whether something floats or sinks isn't about size or weight—it's about "for the same size, is it heavier or lighter than water." Lighter than water floats, heavier sinks; that's density. A foil ball sinks, but pinched into a boat it traps air and the whole thing becomes lighter than water—which is why even a giant steel ship floats on the sea. Twist: stir lots of salt into the water, then drop in the egg or grape that sank before—they'll float now. The water got saltier and "heavier," so it can hold up more.
Materials: a fridge magnet, a stationery magnet, or a small magnet pulled from an old speaker—any will do; plus a handful of paper clips, a few coins, a sheet of paper, and a plastic cup. Lots of ways to play: ① Treasure hunt—walk a magnet around the house and find what it grabs (the iron spoon, screws, the door) and what ignores it (plastic, wood, an aluminum can); ② Action through the air—put a paper clip on the paper, slide the magnet underneath, and the clip "runs" along by itself; ③ Like poles repel—bring two magnets close and feel that invisible "push" shoving them apart.
She's in charge of "carry the magnet and make it grab"—hold up a big fridge magnet and touch it all around the house; it "clicks" onto iron things and does nothing to the rest, and that "it grabbed by itself" magic never gets old. Cheer along with her: "It stuck! ... Not this one~" Her favorite: scatter paper clips on the table and let her "scoop up" a whole dangling string with the magnet, lifting it like a fishing line.
She "explores the limits"—make a list of what the magnet grabs and what it doesn't, then find the pattern (almost everything it grabs is made of iron; aluminum, copper, and some stainless steels don't stick). Challenge: what can magnetism work through? Through paper, through the water cup, through a hand, through a few books? She'll find magnetism passes through non-iron things but gets weaker the thicker they are. Level up: run a maze with the magnet under the paper, guiding the clip on top to the exit.
Big sister is "maze operator," moving the magnet beneath the paper; little sister is "loader and spotter," placing the clip at the start, watching that it doesn't fall behind, and calling "a little left!" (no overlap: fine control under the paper goes to big sister, loading and directing go to little sister). Build a "magnet theater" together: draw a river on the paper, clip a paper-clip onto a paper duck, big sister pushes the magnet underneath to make the duck "swim" across, little sister narrates the story.
A magnet has an invisible "magnetic field" around it that can reach through air, paper, and water to grab iron things, which is why the clip follows along without being touched. A magnet has two ends, one called N and one called S; the same ends push apart and different ends pull together—that pushing and pulling is "magnetism." Twist: tie a paper clip to a thread, pin the other end of the thread to the table, then slowly bring a magnet near to "hover" the clip in midair without touching it. The dangling, floating clip looks like real magic to a child.
Materials: a stiff ruler (or a long stick or chopstick), a round pen or a roll of tape for the "fulcrum," and a few coins or small toys as the weight. Play lever: lay the pen down as a fulcrum, balance the ruler on top like a seesaw, put the weight on one end, and press the other end down to lift the weight—then slide the fulcrum toward the weight and notice it gets "easier." Then play moving things up a ramp: use the ramp (from Activity 1) to push something heavy up onto the table, easier than lifting it straight up. Go hunting for simple machines around the house too: scissors, a bottle opener, a doorknob, a screw—all of them count.
She's in charge of "press down and watch it fly"—put a small toy on one end of the ruler, let her smack the other end, and the toy "boings" into the air. That "I press and it flies" thrill keeps her giggling. Just treat it as a toy-launching catapult; no need to understand the why. Let her play with the ramp too: push a toy car up the slope and let go to slide down, feeling "hard to push up, easy to slide down."
She's in charge of "find the easy spot"—keep the weight fixed and slide the fulcrum (the pen) bit by bit toward the weight, feeling it get easier and easier to press; slide it back to the middle and it gets hard again. Have her state the rule: the closer the fulcrum is to the weight and the farther from your hand, the easier it is. Ask: why can a light person on a seesaw hold down a heavy one by sitting farther out? (Same idea.) She can even count: to lift this stack of coins, how far does the hand-end have to travel at its easiest? (You save force but spend distance.)
Big sister is "engineer"—she sets the fulcrum, adjusts its position, and decides where the weight goes; little sister is "power + counter"—she presses the ruler's end, counts the toys that fly up, and reports "easier or harder this time" (no overlap: tuning the position goes to big sister, supplying force and counting go to little sister). Build a "catapult" together: ruler plus fulcrum, a small ball on one end, little sister smacks the other to fling the ball, big sister slides the fulcrum to adjust range—who can land the ball in a box across the room?
The lever is humanity's earliest "force-saving trick": with a stick and a fulcrum, a small push on the long end pries up a big weight on the short end—the force you save is paid for with "your hand traveling a longer distance." There's no free force in the world. Scissors, bottle openers, seesaws, even doorknobs are all levers. Twist: use a broom as an extra-long lever, wedge one end under a sofa leg, and have the smallest person in the family press the far end—can she "lift" the corner of the sofa all by herself? A firsthand taste of a lever's power.
If you'll do only one, the least-fuss choice is Float or Sink—half a basin of water, toss in whatever small things are lying around, and guess float or sink. Zero prep, zero cost, and cleanup is just pouring out the water. Its best feature: everyone guesses wrong. A big block of wood floats, a tiny nail sinks; a foil ball sinks but a foil boat floats—every surprise gives the little brain an "ooh." The 3-year-old handles the plopping and fishing; the 9-year-old takes on the foil-boat cargo race and the salt-water trick that floats an egg—one basin of water yields several layers of surprise, the gentlest way into this week.