First, lets start of by answering what an OS is. An OS (operating System) is an interface that interacts between the hardware and the programs inside a computer. Some types of operating system’s are Windows, Snow Leopard, and Linux. Now for the good stuff. The tip comes in when you want to run windows inside  Snow Leopard or Linux in windows. Well, there are many paid solutions but there is one that is free. It's called Virtual Box made from Sun. You can install it and use it to run other OS's. Take a look www.virtualbox.org.

Difficulty level: Hard