Kensington lock

It's like a bike lock but for a laptop really

You slot one end into the Kensington lock socket on a laptop, and the other end is secured to a solid surface such as a desk in the office.

Then you can safely leave the laptop on your desk knowing that no-one can take it away, unless they basically rip it out of the socket.

If you have a Pro, then you've a slot for such a lock on the side of your laptop. If you have an Air, then you don't. But there is a new sort of lock made for the Air that fits in the area between the hindges, between the screen and keyboard, essentially. I don't know how it works, but it basically acts as that missing slot, so you can attatch a Kensington lock to it. I do not have one of these items and so cannot speak to how good it is, but the lack of any slot for a Kensington lock has long been an issue with the Airs.