How can I stop a door slamming?

The internal door in my block of flats keeps slamming. For various reasons, I want to stop this happening, but I can't find anything to stop it slamming!
Any ideas what I could use?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).