What you need
- A bucket or large vessel of water
- 10 safe objects: a spoon, a plastic lid, a lemon, a stone, a candle, a coin, a leaf, a matchbox, a steel katori, a ball of foil
- Paper and pen for the prediction chart
How to play
- 1
Make the chart
Draw two columns on the paper: FLOAT and SINK. Your child writes or draws each object in the column where they think it belongs — before any testing.
- 2
Predict out loud
For each one, ask 'why do you think so?' Wrong reasons are fine. You are collecting their current theory, not grading it.
- 3
Test one by one
Drop each object in and watch. Tick the correct predictions, circle the surprises. Surprises are the good ones.
- 4
Break the heavy theory
Most children say 'heavy things sink'. Then a big steel katori floats and a small coin sinks. Let that contradiction sit — do not rescue it too fast.
- 5
Test the shape idea
Take the ball of foil that sank, flatten it into a boat shape and float it. Same weight, same metal, different result. Shape matters, not just weight.
- 6
Ask the real question
'So what actually decides it?' Whatever answer they give, they now own a theory they built and tested themselves.
What your child is learning
Honestly stated — this is what the activity actually builds, not a list of everything it might touch.
- Making a prediction and then testing it — the scientific method in miniature
- That being wrong is useful information, not failure
- Early ideas of density, weight and displacement
- Recording results and reading a simple chart
Every science fair project a ten-year-old will ever do rests on one habit: guess first, then check. This activity trains that habit with a bucket of water and things you already own.
The prediction is not optional
It is tempting to skip the chart and go straight to dropping things in the water. Do not. The prediction is what makes the result mean something. Without it, this is splashing. With it, it is an experiment.
The steel katori moment
The moment to wait for is when a heavy steel katori floats. It contradicts everything the child believed thirty seconds earlier, and you can watch them think. Resist explaining. Ask “how is that possible?” and let them struggle with it for a minute.
Going further with an older child
- Try the same objects in salt water — dissolve four spoons of salt and retest.
- Load the foil boat with coins, one at a time, until it sinks. How many did it hold?
- Ask them to design a boat shape that holds the most weight. Now it is engineering.
Do this outside or in the bathroom. Water will end up on the floor, and that is a fair trade.
Published 6 July 2026 · Suitable for Preschooler, Early schooler, Big kid