Why does a wire get hot when current flows through it?

Current is defined as the flow of electrons. This flow is caused by the electrons within the wire being pulled by electric field. As the electrons travel through the wire they collide with the atoms that make up the wire. During those collisions the electron transfer some of their kinetic energy onto the atoms of the wire. Temperature is defined as the measure of average kinetic energy in the system. Therefore as more electrons collide with atoms, the wire gains more kinetic energy and therefore its temperature increases.