The sink or float experiment is one of the best first science activities for preschoolers because it teaches the scientific method — hypothesis, test, observation, conclusion — in a completely accessible way. Every child can make a guess, every guess is legitimate, and every test gives an immediate, clear result. Here's how to run a great sink or float activity and the science behind why things float.

The Science of Floating

An object floats when it is less dense than the liquid it's placed. Density is how tightly packed the material is — a wooden block has less mass packed into its volume than a metal bolt of the same size, so the wood floats and the metal sinks. The technical term for the upward force water exerts on objects is buoyancy.

For preschoolers, the explanation is simpler: "Heavy things sink. But sometimes even heavy things float — like a big ship — because of their shape." This introduces the idea that shape matters, not just weight, which leads to the boat-building extension activity below.

What You Need

• A clear plastic bin or large bowl filled with water
• 10–20 objects for testing (see list below)
• A simple recording sheet (two columns: "Sink" and "Float" with a picture box for each object)
• Pencil or crayons for recording

Object Ideas by Category

Objects that float: wooden block, cork, plastic bottle cap, orange (unpeeled), dry sponge, foam piece, leaf, popsicle stick, crayon.

Objects that sink: coin, small stone, metal spoon, marble, key, bolt or screw, small battery, glass pebble.

Surprising results (use these to spark discussion): an orange floats in its peel but sinks when peeled (the peel adds volume and air pockets) — this is a great "mystery" object. A ball of aluminum foil sinks, but the same foil shaped into a flat boat floats. A raw egg sinks; in very salty water, it floats.

The Scientific Method in Practice

Before dropping each object, hold it up and ask: "What do you think will happen — will it sink or float?" Record the prediction with a simple "thumbs up/thumbs down" or check mark. Then test. After each result: "Were you right? Why do you think it did that?"

Preschoolers don't need to understand density to engage in this process. They need to practice the cycle of predict → test → observe → discuss. This habit of mind — making a guess and then checking it against reality — is the foundation of scientific thinking.

Extension Activities

Build a Boat

Give children a piece of aluminum foil and challenge them to build a boat that floats AND can hold cargo (pennies or small pebbles added one at a time). This teaches that shape dramatically affects floating — a flat sheet of foil sinks, but the same foil shaped into a boat floats and holds weight. Count how many pennies the boat holds before sinking. Compare results if you try different boat shapes.

Does Water Type Matter?

Test objects in plain water, then in very salty water (dissolve ½ cup salt in a gallon of water). Objects float more easily in salt water because salt water is denser than plain water. A raw egg sinks in plain water and floats in salt water — a classic and memorable demonstration. This is why people float more easily in the ocean than in a lake.

Liquid Layers

Carefully layer three liquids in a tall clear glass — honey on the bottom, water in the middle, vegetable oil on top. Drop objects in and see where they settle. Objects denser than honey sink to the very bottom. Objects denser than water but less dense than oil float on the water-oil boundary. This visually demonstrates that liquids themselves have different densities.

Sorting by Weight vs. Sorting by Float

Before testing, have children sort the same objects by weight — heaviest to lightest — using a simple balance scale or their hands. Then compare the weight sorting to the sink/float results. Are the heaviest things always the ones that sink? (No — shape and material both matter.) This builds critical thinking about what actually determines floating.

Recording Results

For preschoolers who can't write yet, draw a simple two-column chart: one column with a water symbol at the top (for float), one with an arrow pointing down (for sink). Children draw or stamp the objects into the correct column. Four-year-olds and older preschoolers can write the object name or glue a photo of each object into the chart.

Group Activity Setup

For classroom use, set up the water bin as a center. Keep the object basket beside the bin. Provide prediction cards — a small card with a "thumbs up / thumbs down" choice — for children to vote before each test. After testing 10 objects, gather the group and discuss: "Which objects surprised you? Why do you think the cork floated even though it's hard?"

Frequently Asked Questions

What age is sink or float appropriate for?

Children as young as 2 can engage with the basic activity — dropping objects in and watching what happens. The prediction-and-recording component is more meaningful for ages 3–4+. The extensions (salt water, liquid layers, boat building) are best for ages 4–6.

How do I explain density to a preschooler?

Don't explain density directly. Instead, use the language of "packed tightly" and "lots of stuff inside vs. mostly air." A brick is "packed with lots of heavy stuff." A foam block is "mostly air and a little foam." Objects with mostly air inside float. This builds an intuitive understanding that accurate vocabulary can be added to later.

My child just wants to splash. Is that okay?

Splashing and free exploration always come first. Let children interact with the water freely for 5–10 minutes before introducing the structured activity. Children who have had free exploration time are much more receptive to guided observation afterward.