Oral injuries are often painful, and should be treated by a dentist as soon as possible. If you have knocked out a tooth, these tips may be able to save it:

Rinse, do not scrub, the tooth to remove dirt or debris
Place the clean tooth in your mouth between your cheek and gum or under your tongue
Do not attempt to replace the tooth into the socket as this could cause further damage
Get to the dentist. Successful re-implantation is possible only when treatment is performed promptly
If it is not possible to store the tooth in the mouth of the injured person, wrap the tooth in a clean cloth or gauze and immerse it in milk.