A tooth can be knocked out for a number of reasons: often a blow to the mouth, or an accident involving the face. It is possible to replace the tooth in the socket successfully if the right action is taken as soon as possible.
If a tooth has been knocked out and you are bleeding, don't panic. Get a clean handkerchief and fold it up, then hold it over the socket and bite down. Keep your jaws together to apply pressure. If you need something for the pain, don't take any medication containing aspirin as this can encourage further bleeding. Do not apply clove oil to the wound.
If you still have the complete tooth, it needs to be replaced in the socket as soon as possible, ideally in under 30 minutes. But teeth have been successfully replaced up to 60 minutes after being knocked out. Avoid handling the root of the tooth. If it is very dirty, rinse it with milk and wipe it with a clean cloth. Do not clean it with disinfectant or water or let it dry out. Your tooth has more chance of survival if you keep it in your cheek until you can get emergency dental treatment. This will keep the tooth in its most natural environment. If this is not possible, keep it in some milk.
There are ways to prevent teeth getting knocked out, for example wearing a mouth guard - particularly for sports.
Your dentist can have one made for you by taking an impression of your teeth and sending it to a laboratory. The laboratory then makes the mouth guard so that it fits your mouth exactly. Mouth guards can be clear or coloured - for example in the colours of the team kit if you want to wear one while playing sport.