Hard-Boiled or Raw?

Have you ever made hard-boiled eggs, then mixed them up with raw eggs, so that you couldn't tell them apart? Even if you haven't done this, is there a way to distinguish them? They certainly look the same on the outside. Can we tell what the inside is like without breaking the egg?

Yes, we can! Ask yourself - what is inside a raw egg? There's a yolk and some runny egg whites - but everything is a liquid. Now what about a hard-boiled egg? Everything has turned to solid because it was cooked (more on that later!). We can make observations about the behavior of these eggs that tell us what is inside.

Take one hard-boiled and one raw egg. Clear an area on a countertop, lay the hard-boiled egg on its side, and give it a spin. It will wobble a little, but will keep turning. Now place your index finger lightly on the egg just long enough to make it stop. Remove your finger and watch what happens. Big deal - the egg remains stopped.

But now, try the same experiment with the raw egg (make sure it doesn't spin off the countertop). What happens when you remove your finger? The egg starts to turn again! Why?
Remember that the inside of this egg is filled with a liquid. When you put your finger on the shell, you stop the shell, but the liquid keeps moving. When you take your finger away, the moving liquid gets the shells going again. In the hard-boiled egg, everything stops when you stop the spinning, and the egg doesn't start turning again.

This effect actually has a fancy science name - inertia (pronounced in-er-shah), which is the tendency of moving things to stay in motion. Think about riding in a car when the driver suddenly puts on the brakes, you are thrown forward because of your inertia. At first, you and the car are both moving at the same speed. But when the brakes applied, the car stops but you don't. Can you think of any other examples of times when you feel inertia?