The plastic egg is partially filled with tiny beads inside. This is the kind of egg used a shaker for musical purposes.

Place the egg on a gently sloped surface, and it doesn't roll down. If you tip the surface enough, it will roll. The beads inside the egg stabilize it.

list materials here:

• egg shakers come pre-made, though you could make your own using hollow plastic eggs and filler material.
• a piece of carboard and something to prop it on.

If you're making your own shakers, experiment with different fillers. Glue the shakers shut to keep kids from opening them and getting the filler everywhere. Be sure to wipe off excess glue so that you have a nice smooth surface - a bumpy surface could be the reason the egg doesn't roll, and we don't want to affect the experiment like that.

Play around with different angles for the slope, find an angle that the eggs just barely don't roll. Then find an object of the correct height to prop the slope on.