Egg shells can seem so fragile, yet they're remarkably strong. If your child has a science fair coming up, encourage him to choose an experiment that features eggs. These experiments have the sort of razzle dazzle that will impress viewers and make them say, "Wow! I never knew that about eggs."
Eggs float in saturated salt water, while they'll sink in regular water. Your child can demonstrate this by showing a floating egg in one glass container and a sunken egg in another container. According to the Steve Spangler Science website, you can also make the egg appear to float halfway in a glass of water by mixing a kosher salt into half a glass of water, then pouring plain water down the side, taking care not to mix the two types of water.
Vinegar dissolves the hard shell of an egg, but it leaves behind the thin membrane, allowing people to see through the egg. Your child can prepare a few "naked eggs" before the science fair for viewers to see and touch. However, she'll probably also want to place an egg in vinegar at her display table, as the process of the vinegar dissolving the water is fun to watch -- the egg bubbles and spins around.
Egg Shell Strength
Your child can demonstrate that the way an egg shell breaks is by applying uneven force. When he takes an egg in his hand and squeezes it evenly all around, it will not crack. However, if he applies some sort of additional pressure in one spot, such as having a ring on his finger, the egg will break. If he really wants to impress the judges, have him stand with each foot on a dozen eggs. If he's applying the force evenly all around, the egg shells shouldn't break, even underneath his weight.
Traditionally, you'd find geodes inside rocks, but if your child wants to demonstrate how crystals are formed, doing it inside egg shells is a pretty way to display them. The Happy Scientist website suggests making an epsom salt and water solution, then pouring it into broken and clean egg shells and adding food coloring. After a few hours, she'll have crystals. She can also demonstrate different types of crystals, such as ones made from salt, sugar or calcium chloride.