Sink or Float


This week’s activity, “Sink or Float,” allows your child to explore buoyancy and density, and experiment with common objects to find out which sink in water (fall to the bottom) and which float in water (stay on top of the surface). 

Safety: Please don’t allow children to put small objects into their mouths, and don’t allow children to use any electronic items in or around water. 

What we’re learning and skills we’re building 

  • Buoyancy – the ability of an object to float in water or air 
  • Density – the measurement of how compact an object is 
  • Observation – watching and learning what happens to objects when they’re placed in water 
  • Sorting – systematically categorizing objects according to whether they sink or float in water 
  • Hypothesizing – guessing what will happen to each object based on what’s already known about it 


  • A large bowl or a small tub of water 
  • Paper and markers (to create “sink” and “float” labels) 
  • Towels or 2 bowls (to sort the objects on/in)  
  • Common household objects (NO electronic items; see suggestions below)

Collect items that are safe and ok to get wet, such as:

Cups, bottle caps, keys, leaves, paper, cat toys, corks, straws, cutlery, cardboard, TP tubes, pens or pencils, rocks, paper towel, Skittles or other candy, and packing peanuts, for example!



Have your child place each object one by one into the bowl of water. Let the objects to sit in the water for a little while (30 seconds to 1 minute). Your child can move the objects around in the water, but not too much. Some objects that float at first might begin to sink! 


A younger child can simply test out the objects and and see if they sink or float. If your child is a little older, ask questions and encourage conversation. Before they test the objects, ask: “Which ones do you think will float?” “Which do you think will sink?” “Why?” (Scientists call this making hypotheses.) Then, while they’re testing the objects, talk together about what’s happening to each in the water. Ask: “Are you surprised by what happened to any of them?” “Are the outcomes what you expected?” “Why do you think some didn’t work as expected?”


Once the objects have been tested, your child can sort them into groups according to whether they sank or floated. Ask: “What do all of the items in the first group have in common?” “Are their shapes similar?” “Are they made out of the same material?” “Do they weigh about the same?” “What about the second group?”


More Learning

  • Some children (and some adults, too) mistakenly think heavy items sink and lightweight items float, but that’s not always the case (although you can find lots of examples that fit this hypothesis).
  • Ask your child, “Can you think of a very heavy object that floats?” (Boats, both small and large, are examples of this. Even huge battleships!) 
  • Floating happens when an object is less dense than the water. 
  • You can make a heavy object float by changing its shape. Here’s a fun demonstration: roll out two balls of modeling clay, each the same size. Do they weigh the same? (Yes.) Drop one into water – it sinks! Now, flatten the other ball into a leaf or a boat shape – it floats! You’ve changed the specific gravity of the second ball of clay. This is why big ships float.