First-aid guides often tell us to transport a tooth to the dentist in milk — or better yet, tucked under your tongue where it can stay covered in saliva. But it's OK to put the tooth back in its socket, and in fact, that's what dentists recommend.
Photo by Partha S. Sahana.
You'll want to rinse the tooth off first if it's dirty (but only take a few seconds to do this, and don't touch the root). If you're able to put it back in its socket, bite down on something soft, like a handkerchief, and head straight for the dentist. If you can't get it back in, keep the tooth covered in saliva — either under your tongue, or spit into a carrying container.
The dentist will probably give you a course of antibiotics, and recommend a root canal to combat infection. None of this is recommended for a baby tooth, though — tell the kid to put it under their pillow and enjoy an early visit from the tooth fairy.
Avulsion First Aid [Dental Trauma Guide]