Materials: an ice tray (or little cups), water, a drop or two of food coloring (optional), a clear glass, and a kettle of just-boiled water. The night before, freeze a few cubes: half plain, half with coloring. On the day, play in two steps—first watch ice melt into water, then (adult) pour hot water to see the white steam.
Her job is "hold the ice" and "wait for it to melt": squeeze a cube in her palm and feel "so cold," "so slippery," "it's getting smaller," and watch a little puddle appear in her hand. She'll pick it up and put it down again and again—that's her body learning that a solid turned into a liquid. Keep a small towel handy to dry her hands.
Her job is "time and record": take two cubes the same size, leave one on the table and squeeze the other in her hand (or drop it in warm water), guess which melts first, then time it on a phone. While watching the steam, have her observe and answer: what's that white cloud at the rim? (Hint: when it meets cold air it turns back into tiny droplets, which is why we call it "water vapor.") Have her trace the ice → water → vapor "three states" path out loud.
Big sister is "timekeeper," calling "Go!" and "That one's melted!"; little sister is "ice manager," fetching cubes from the bowl and lining up the two "contestants." One uses her head with the stopwatch, one uses her hands setting up the ice—see which cube wins the melting race.
It's the same water all along, just "dressed" differently: cold and solid is ice, soft and flowing is water, hot enough to fly off and turn invisible is vapor. A warm palm and hot water both add heat to the water—change the temperature and you change its form. Twist: put the melted water back in the freezer, and tomorrow it's ice again—showing her this road runs both ways, with water circling endlessly among its three states.
Materials: a coin (a large one is best), a cup of water, a dropper or cotton swab (no dropper? plug one end of a straw to move water), a few grains of ground pepper, a toothpick, and a touch of dish soap. Lay the coin flat on the table and start "stacking" water on top.
Her job is "count the drops": add water one drop at a time with the dropper or swab while you count together "1, 2, 3…," watching the surface slowly bulge into a clear little hill, until it finally spills with a "pop." That moment when the dome swells and bursts will make her laugh—let her do it over and over.
She takes the "max-drops challenge": guess how many drops fit on one coin without spilling, write the guess down, then carefully drip and count until it breaks, and check how close she was (often thirty or forty—way more than people expect). Bonus magic—scatter a layer of pepper on a bowl of water so it floats, then dip a toothpick in a tiny bit of dish soap and touch the center; watch the pepper dart away. Ask her: what pushed the pepper off?
Little sister is the "dripper," adding one drop at a time; big sister is the "recorder," counting and watching for when the dome breaks. When it breaks, big sister performs the "fleeing pepper" magic trick for little sister, who scatters the pepper on the surface and gasps. One tests patience, one does magic—take turns on stage.
Water molecules hold hands and tug on each other, pulling the surface tight into an invisible "skin" (called surface tension), which is why water can pile into a hill on a coin instead of spreading flat. Dish soap "cuts" that skin and weakens the pull; once the skin loosens, the unbroken water elsewhere drags the pepper outward, so it flees. Twist: try floating a paperclip on the water—lay it down gently (or rest it on a scrap of paper; when the paper sinks the clip stays). Metal is heavier than water yet floats, held up by that same skin.
Materials: two identical cups (or one tall, one short), a length of soft tubing (a straw is too short—use a flexible plastic tube, or join a few straws), water, and a drop or two of coloring to make it show. Fill one cup and set it up high (on a book), put an empty cup down low. Fill the tube with water, pinch both ends, put one end at the bottom of the full cup and the other into the empty cup, and let go—the water flows over by itself.
Her job is "watch the water run" and "hold the cup": help steady the empty cup and watch the colored water climb all the way through the tube and pour into the empty one. Water "crossing the mountain" to move itself is pure magic to her, so let her lie by the cup and watch her fill, then help you carry the cups back and forth to refill.
Her job is to "start the siphon" and find the rule: get it going with the fill-the-tube method, then experiment—lift the empty cup up to the same height as the full one; what happens? (It stops.) Lower the empty cup more; does the flow speed up? Let her discover the key: the outlet must be lower than the water surface for the water to flow. Challenge her to explain it to her little sister in one sentence.
Big sister is the "engineer," setting cup heights and starting the siphon; little sister is the "water-level watcher," calling "This cup's empty!" and "Switch sides!" When it drains, the two team up to swap the cups and send the water back once more. One adjusts heights, one reports levels—together they make the water shuttle back and forth several trips.
The water on the full side gets pulled down harder (the downhill stretch of water in the tube is heavier), like a chain made of water: the heavy end drags the light end down with it, so the water climbs over the rim and flows to the low side—that's a siphon. Aquarium water changes and plant-watering bulbs all use it. Twist: lower the empty cup more and more and watch the flow speed up; raise it above the full cup's surface and the water stops at once—lift and lower, and the rule "it must go downhill" becomes obvious.
Materials: an empty plastic bottle, a thumbtack or toothpick, water, a strip of tape, and a basin (or do it in the bathroom/on the balcony). An adult first pokes three holes down one side (high, middle, low), covers them with tape, fills the bottle, then peels the tape off to see three streams shoot out.
Her job is "peel the tape" and "pour the water": help fill the bottle (you steady it), then rip off the tape and watch three streams shoot out with a "psshh." Which one goes farthest? She reaches out to catch the streams and stomp in the puddles—getting wet is the point of this one.
Her job is "observe and explain": look closely at the three holes—does the bottom one shoot farthest or shortest? (Farthest.) Have her guess first, then verify, and think about why. Bonus: screw the cap on tight, then loosen it, and watch the flow change (tight cap weakens or stops the flow, because air can't get in). Challenge her to draw a diagram marking how far each of the three streams reaches.
Big sister is the "commander," deciding which hole to cover and how full to fill; little sister is the "tape-peeler," ripping on command. Together they watch which stream goes farthest and race to stomp the spot where the farthest one lands. When it runs dry they refill together and play until they're both soaked.
The deeper the water, the more water sits on top pressing down, so the harder it pushes out—which is why the lowest hole has the highest pressure and shoots farthest. Your ears feel the squeeze deeper underwater, and dam walls are built thickest at the base, for the same reason. Twist: try the bottle full versus half-full and compare the same hole's reach—with shallow water that stream droops right away, making "deeper means stronger" plain to see.
Too tired? Do the Water Dome on a Coin—one coin, one cup of water, one cotton swab, and a paper towel on the table, and you're set; no hot water, nothing sharp, the easiest to manage. Watching the water bulge into a clear little hill drop by drop and then "pop" holds both kids spellbound, and a spill is just a small wet patch to wipe up.