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Up or Down?

Students learn how to make predictions about an object’s density and how to test their predictions by seeing if it sinks or floats.

Objects that are more dense (heavier for their size) than water will sink, and objects that are less dense than water will float.

Density is measured as mass/volume and because a litre of pure water has a mass of 1 kilogram, water has a density of 1.0 kilogram per litre. 

Objects with a density less than 1 kg/L will float, and objects with a density higher than 1kg/L will sink. 


  • Predict, test and explain the relative densities of everyday objects through investigating their interaction with water


  • Per Demo or Group:
    1 Bucket or large container full of water
    1 “Up or Down” prediction card
    6 or more various objects such as the following: coin, grape, apple, LEGO piece, cork, styrofoam, popsicle stick, candle

Key Questions

  • Why do some objects float and others sink?
  • Does weight matter? How do you know?
  • Does size matter? How do you know?
  • Does shape matter? How do you know?
  • How could we test out hypotheses about what makes something float or sink?
  • Which objects are denser than water? Which objects are less dense than water?

What To Do


  1. Fill 4 or 5 buckets with water and set them up at stations around the room.
  2. Place at least 6 different objects by each bucket.


  1. Divide the students into 4 or 5 groups, each assigned to one bucket station.
  2. Ask students to study the objects at their station and to predict which will sink and which will float in the bucket of water. Ask them to write down their predictions.
  3. Students then test each object individually and note down their observations.
  4. Once all the objects have been tested, ask the groups to discuss their results and come up with an explanation of why some objects float and some sink.


  • Can we change a sinker to a floater and vice versa?
  • What kinds of changes will cause a change from floating to sinking, or vice sersa?
  • Challenge students to find or create a “flinker,” something that doesn’t quite sink and doesn’t quite float.