When you wear a sweater, the sweater rubs against your inner clothing as you move your body. The rubbing causes positive electric charges to gather on one side and negative electric charges on the other. When you take off the sweater, the negative electric charges are drawn toward the positive side and electricity flows. The crackling you hear is the sound of the electricity flowing. And the tingling you might feel comes from touching the electricity. This kind of electricity is called static electricity.