Sink or Swim STEM Challenge

Have you ever noticed how some objects float in water, while others sink? Boats float because of their shape, but once that shape is damaged, they sink. Some materials, no matter what shape or size, will float in water. For example, loggers who cut down trees would transport those logs weighing thousands of pounds down the river by floating them. How could something so heavy still float in water?

The answer has to do with density. Density measures how closely packed together the atoms of different substances are. Different substances have a specific density that can be measured by dividing mass by volume. Water has a density of 1 g/cm3, which means that every cubic centimeter of water (about the size of a sugar cube) weighs 1 gram. Anything with a density lower than this will float in water, and anything with a greater density will sink.

Sink or Swim STEM Challenge

How to do the Sink or Swim STEM Challenge

The Challenge: Investigate what kind of materials are denser (sink) and less dense (swim) than water. 

Materials: A small tub or bucket of water, items made of different materials (metal, plastic, wood, ice, food items like apples or grapes, items from nature, etc.). Gather at least 10 different items to test.

Challenge Criteria: Examine each item before dropping it in the water.  Make a prediction about whether you think it will sink or float.  Then test your prediction.  Note: some items will appear to float because of their shape (like a leaf on the water).  Make sure to submerge the item first, then see whether it floats back to the surface or not. 


What are some shared properties of the “sink” items? What are some shared properties among the “swim” items? If you didn’t yet test ice, do so now. Ice is just frozen water, but what more do you know about it based on this experiment?

Challenge extension

Try to use one of the “swim” items to keep one of the “sink” items from sinking, like a life raft!


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