Laptops start overheating when there isn't sufficient cool air getting to the CPU. I will show you how to stop your laptop from overheating.

Despite their name, laptops are not supposed to be operated on your lap. The reason for this is that a laptop usually draws cool air in through an air intake in the base. If this is blocked then the laptop will quickly overheat and may shut down. 

A laptop should also not be used on a soft surface like a duvet or carpet. Over a period of time dust and fluff will be drawn into the laptop and block to copper fins on the air vent on the side (or rear) of the laptop as can be seen in the photo.

If this happens then the laptop cooling fan will have to work harder to expel the hot air until your laptop starts to overheat. There is no easy way to remove this dust and fluff from your laptop other than to gain access to the fan housing and fan on the motherboard.

You can see a brief explanation of how the cooling system works in a laptop from the photo below.

Laptop cooling systemThe CPU (central processing unit) and the GPU (graphics processing unit) are the 2 chips that generate the most heat within a standard laptop and so need to be cooled.

In the photo they are connected by heat pipes to a finned grill at the air output. The fan forces air through this grill to keep the laptop cool at at a safe operating temperature. 

As stated earlier, if this grill becomes clogged with dust and fluff, the laptop will eventually overheat.

To further assist the cooling of the laptop, there is a fine layer of thermal paste between the  CPU/GPU and the heat pipes to help transfer the heat away from them. Constant overheating will also tend to 'dry out' the thermal paste which will make the overheating even worse. It can be replaced but will involve further stripping down of the laptop.