This is a nice prank to change mouse clicks. You can change mouse clicks in absence of your friend and then can see him struggling on using mouse.
because it will change the actions of click, Right click will work like left click and left click will work like right click.
Follow these steps to change mouse keys:
1). Open control panel.
2). Double click on Mouse in control panel, This will open mouse properties window.
3). Now in mouse property window go to buttons tab and click on switch primary and secondry buttons under mouse configration.
4). Click OK.
To make it normal, follow these steps again and unclick switch primary and secondry button.