This morning I asked myself a simple question:  “Why, when I’m carrying the cat’s full water bowl up the stairs, does the water slosh over at the top step?”

A question like this can often generate some interesting science.


  • a couple of sturdy mixing or soup bowls with sloped sides, at least two different sizes
  • a straight-sided bowl, such as a casserole dish
  • water
  • metronome (optional)
  • water-proof clothing such as a rain jacket (if it is cold out or if the children can’t change clothes if they get wet.)
  • measuring cups (optional)

This is probably best done outside where a little spilled water won’t be a problem. First fill one bowl with water and see what happens when you walk with it held out in front of of you. Does the water begin to move back and forth? What happens when you stop?

Now compare that with a bigger or smaller bowl. Does more water go out over the edge with a big bowl or a small bowl? What about slope sides versus straight sides?

If you have a metronome, try walking at a constant slow pace versus a constant fast pace. See any differences?

You can actually make this more scientific by measuring the amount of water you put into the bowl and how much you have at the end with liquid measuring cups.

What we are seeing is the resonance, or the swing, of the water. When the swing gets big enough, over the edge it goes. This may not seem so important until you realize that the same sorts of forces are acting on the waves and tides of the oceans.

My son suggests I carry the water up the stairs in a straight-sided pitcher. Based on your studies, what do you think of that idea?