Father and child put a hard-boiled egg into a glass jar containing about four inches of fresh water. Ask someone to pour in more water. The egg stays at the bottom of the jar. Pour the water out of the jar. With magic words, add magic water from another pitcher slowly the egg rises to the top of the jar.
Explanation: The second pitcher contains a strong solution of salt water. The density of the salt water is greater than that of the egg, and so the egg floats.