I was just listening to one of my operators talk to a caller who had a tooth knocked out, and I reminded him that many times a tooth can be saved when it has been completely knocked out and is still intact. If the tooth is broken off from the part below the gum, it's no longer possible to save it, but if you can see that it is the whole tooth, you have a chance.
The most important thing to remember is to get to a dentist with the tooth right away, and to wrap it in a small cloth soaked with milk. This will preserve the tooth and keep it from dehydrating, and often a dentist can put the tooth back in place and the root will take hold.
So if you knock out a tooth somehow, make sure you find it, and get to your dentist (or one of ours) immediately.