Science Fenns K-5Grades K to 5
A clear tub of water with a cork floating, a metal bolt sunk on the bottom and a grape bobbing in between.

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Density: Sink or Float

Same size, different heaviness.

Lesson 9 of 40

Try this first

Before we start

Today’s story

Density: Sink or Float

Carlos stands at the stock tank on a hot day. He tosses in a stick and it floats. He tosses in a rock and it sinks. The rock is smaller than the stick. So why did the small one go down.

The answer is density. Density tells how much matter is packed into a space. A rock packs a lot of matter into a small space. A stick packs less. Something with more density than water sinks. Something with less density than water floats.

Big does not mean sink. A huge log floats. A tiny pebble sinks. Heavy for its size is what counts. Here are the steps.

See it

Five steps. Every time.

    Hands on

    Sink or Float Test

    20 minutes · Do this with a grown-up nearby

    You need

    • A tub or large clear bowl of water
    • A cork
    • A coin
    • A wooden pencil
    • A grape
    • A plastic spoon
    • A paper clip
    • Paper towels

    Do this

    1. Line up the objects.
    2. Predict sink or float for each.
    3. Drop the first object gently into the water.
    4. Watch and write what happens.
    5. Remove and dry it.
    6. Repeat for each one.
    7. Check which predictions were right.
    Write or draw: A table with Object, Prediction, Result and Match yes or no.

    Watch for: Notice that some objects sink slowly. A grape or a plastic spoon can surprise you. Drop the paper clip in. Do not lay it on top.

    Safety: Wipe up spills right away and keep water away from outlets.

    Same every time

    Three rules

    Rule 1Size alone does not decide.

    A big log floats and a small bolt sinks.

    Rule 2Predict, then test.

    A guess shows what you think.

    Rule 3Compare to the liquid.

    Something can float in one liquid and sink in another.

    Grandpa Fenn’s notebook

    Say it like a scientist

    density
    How much matter is packed into a space.
    float
    To stay on top of a liquid.
    sink
    To go to the bottom of a liquid.
    buoyant
    Able to float.

    Poppy’s quiz

    Six questions

    Tap an answer. Green means right. You will see why.

    Talk about it

    Say it out loud

    • How do you know an object is denser than water?
    • Where have you seen things float or sink at a creek, tank or pool?
    Did you know

    Ice floats on liquid water because it is a little less dense than water. That is unusual for a solid.

    Someone who does this

    A ship designer uses density to be sure a ship floats.

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