• metal paper clips
• a wire coat hanger(optional)

Bend one of the paper clips to straighten it a bit. Now bend the metal back and forth twice. Feel of the metal where it bent. It should feel warm. If you can't feel the warmth, try it again and this time touch the metal to your lip or to the back of your hand.

Bend the metal in the same place a few more times. Count how many times you have to bend it before it breaks. As soon as it breaks, touch the broken end GENTLY to feel how warm it is. Be careful not to scratch yourself on the sharp metal.

Try the same thing with a piece of wire from the coat hanger. How many times do you have to bend it before it breaks. How hot does it get?

Why does the metal get hot? As you bend the metal, the molecules in the metal rub past each other. This produces heat in much the same way that your hands get hot when you rub them together. As you bend the metal, you weaken the metal by breaking some of the bonds that hold the molecules together. The more you bend it, the more bonds you break. Compare this with a piece of plastic. When you bend the plastic back and forth, does it get hot? Does it break? Probably not. The molecules in the plastic move more easily, so it does not get hot and does not break as easily. You can also try different kinds of metals, from copper wire to aluminum foil. The kind of metal and how thick it is will make a big difference.

When you are building airplanes, cars, bridges, buildings, etc., you have to remember that metal fatigue can weaken the structure. If you don't plan for this, your bridge may wind up floating down the river.