You will literally get the shock of your life if you do. Those who have tried to do so, perhaps in a misguided attempt to free a stuck slice, will readily testify. A burst of electric energy flowing through your body is not the only hazard you will be exposing yourself to when you put a knife in the toaster. If the toaster is still on, you also run the risk of heating the knife and burning your hand in the process. The possibility of this happening is greater if the knife has a metallic handle as most metals are good conductors of heat. Besides the danger to your person, you also risk damaging your precious toaster. The elements or the resistive wires in the toaster that enable it to convert electrical energy into heat energy can be ruined when you short-circuit the regular flow of electrical current with your knife. If you have to reach into the toaster slots to remove stuck food, make sure the toaster is switched off at the power supply. Unplug it from its slot on the power strip or surge suppressor if it is plugged into one. Then use chopsticks or tongs made from a non-conductive material to reach into the slots.