Why does my phone heat

@martinsasu we need to know what your exact model is. How hot is to hot? Besides keeping your phone protected from any ambient heat i.e. direct sunlight etc, is the number one thing to do. After that, you want to change some settings. Lower your screen brightness manually if you do not need it at 100%, and set the screen timeout to a shorter duration. Turn off GPS, Bluetooth, Wi-Fi hotspots, and camera sensors when you are not actively using those. Of course, never charge a hot phone. If the phone is already hot from the sun or heavy use, let it cool down to room temperature before plugging it in.

There are only a few things that can make your phone really hot. One is a bad battery and the other reasons are related to components on the main board. Could be an overtasked processor or even a short-circuit on the main board somewhere. If your phone functions without any other issues but the overheating, I'd start by replacing the battery.