The fan on your laptop pulls in air and blows it out over the heatsink fins to keep it cool. Over time a lot dust, dirt and lint eventually clogs the fan blades and the heatsink fins. It usually takes about a year for this to happen if you don’t take preventative measures all along to keep it clean. Once it’s clogged inside, you’ll need to take it apart to clean it. If you don’t feel comfortable taking your laptop apart you should take it to a computer repair shop. If the computer is new, or after you’ve had it cleaned, you can start keeping it clean by using a can of compressed air which you spray into the vents on the laptop periodically. Another way to keep your laptop clean, and therefore cool on the bottom, is to always use a laptop desk that has fans in it. A laptop desk is especially useful when you are using your laptop in bed to keep the lint and dust on the bedding from being sucked into the laptop vents. All laptops naturally get a little warm on the bottom but if yours is exceptionally hot and you hear the laptop fan running constantly, you most likely need to have the fan cleaned. When a laptop gets too clogged with dirt, the laptop will shut itself down shortly after start up in order to protect the components inside.