Wheelies
Most kids get restless just riding their bicycles around and eventually start trying out new things, one of these things being wheelies. Now just stick 750 cc's between their legs and things become interesting! A simple understanding of physics can make things possibly a lot cheaper and a hopefully a lot less painful!!
Torque and friction are 
.
the elements that actually pick the motorcycle up, when a rider gives a bike some gas the back wheel begins to turn. Friction between the back wheel and the ground keep the tire from spinning and set the bike in motion. If more gas is given the torque will become great enough to lift the front tire off of the air.
Then it becomes a ballancing act, every thing has a center of mass, meaning a certain point on an object where gravity acts, this includes motorcycles and in order to keep from tipping over the rider must know where the center of mass is and use that information to his advadtage. He or she must keep the center of mass in front of the back tire, if he or she can do this then the bike will not tip over, if not then the picture above illustrates what will happen.