[dentist90]

Immediate replacement won't keep the pulp (nerve) itself alive, but the root is covered in live cells that attach it to the bone. If the tooth dries out these cells will die and reimplantation will not work. Putting teeth in plain water is a no-no as well as this will kill these cells. Immediate replacement is ideal, hopefully you can figure out which tooth goes where. Saline would help keep the root surface vital for an hour or two in a pinch. Even milk is better than nothing if you can't figure out how to get tooth back in.
Teeth that are knocked out will need root canal treatment after they are reimplanted, but that is preferable to bridges or implants.

edit: the above doesn't apply to teeth that are broken off, only if knocked clean out with their root.