Science activity
Sink or Float
Gather objects from around the house and predict: will it sink or float? Test each one in a tub of water and sort into two groups!
Materials and setup
- large bin or tub
- water
- assorted household objects
- clay
- paper
- pencil
Parent note
Fill the water bin in advance and place it on a waterproof surface or do this activity in the bathtub. Gather a variety of objects with different densities beforehand. Have a towel nearby for spills.
How this changes by age
Pre-K (ages 3–4)
Gather objects from around the house and predict: will it sink or float? Test each one in a tub of water and sort into two groups!
Steps
- Fill a large bin or bathtub with water.
- Collect 8-10 objects: rock, leaf, coin, crayon, sponge, toy car, apple, block.
- Hold up each object. Ask: 'Do you think this will sink or float?'
- Drop it in and watch! Sort objects into a 'sink' pile and 'float' pile.
- Talk about what you notice: some things are packed tightly and sink (like a rock) — others are packed loosely with air inside and float (like a sponge or a boat). It is NOT about heavy or light — a BIG heavy ship floats and a SMALL pebble sinks. It is about how tightly the stuff is packed compared to water.
- Tell a grown-up one thing that surprised you.
- Make a two-column crayon chart: things that floated / things that sank.
Learning objectives
- Make predictions before testing (hypothesis formation)
- Sort objects into two categories based on observed results
- Begin to identify patterns in which objects sink vs. float
Kindergarten (ages 5–6)
Predict, test, and record whether objects sink or float. Explore WHY some things float by testing different shapes of the same material (clay ball vs. clay boat).
Steps
- Set up a water bin and gather 10-12 test objects of various materials.
- Create a chart with columns: Object, Prediction (sink/float), Result (sink/float).
- Predict and test each object, recording results.
- Now take a ball of clay (sinks) and shape it into a flat boat. Does it float now?
- Discuss why shape matters: spreading weight out over water helps things float.
- Tell a grown-up one thing that surprised you.
Learning objectives
- Formulate and test predictions using a scientific method approach
- Record experimental results in a simple chart
- Understand that both material and shape affect whether objects float
Early elementary (ages 6–8)
Investigate density and buoyancy: test objects in different liquids (water, salt water, oil), build boats to hold cargo, and measure how much weight your boat can carry before sinking.
Steps
- Test 10 objects in plain water. Record whether they sink or float.
- Now add salt to the water (3 tablespoons per cup, stir until dissolved). Retest the same objects. Did any results change?
- Research: density is mass divided by volume. Objects less dense than water float. Salt water is denser than fresh water, so more things float in it.
- Build a boat from aluminum foil. Test how many pennies it can hold before sinking. Record the number.
- Redesign your boat to hold MORE pennies. What design changes helped? Record your best result.
- Write a report explaining: What is density? Why does shape affect floating? Why do things float better in salt water?
- In one sentence, tell a parent or sibling what surprised you today.
Learning objectives
- Understand density as the relationship between mass and volume
- Compare buoyancy in fresh water vs. salt water through experimentation
- Apply engineering design process: build, test, redesign, and improve
Upper elementary (ages 8–10)
Conduct a comprehensive density investigation: calculate density of objects, build a density column, explore Archimedes' principle, and design a vessel optimized for maximum cargo capacity.
Steps
- Calculate the density of 5 objects: measure mass with a kitchen scale (grams) and volume by water displacement (mL). Density = mass / volume.
- Build a density column in a tall clear container: honey, corn syrup, maple syrup, milk, water, vegetable oil, rubbing alcohol. Predict the order before pouring.
- Drop small objects into your density column. Where does each one settle? This shows its density relative to the liquids.
- Research Archimedes' principle: an object floats when it displaces water equal to its weight. Explain in your own words.
- Design and build a cargo ship from recyclable materials (foil, plastic containers, cardboard sealed with tape). Optimize for maximum penny-carrying capacity.
- Write a scientific paper: explain density, buoyancy, and Archimedes' principle. Include your density calculations, density column observations, and boat engineering results with diagrams.
- In one sentence, tell a parent or sibling what surprised you today.
Learning objectives
- Calculate density using mass and volume measurements
- Build and explain a density column demonstrating relative densities
- Apply Archimedes' principle to explain buoyancy and design floating vessels
Safety and evidence note
Read the full activity before beginning. An adult should supervise tools, heat, food, outdoor work, movement, and experiments as appropriate. Completion records that the activity was done; the child’s explanation, work sample, photo, or demonstration is stronger evidence of learning than a completion check alone.
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