Just the other day my cousin had a traffic accident. She fell of her 2 wheeler, and though she escaped serious injury one of her tooth came out.
Luckily for her she had the presence of mind to notice it and she immediately carried it to the nearby dentist who fixed it back and wired it in. Hopefully in the next 2-3 weeks the tooth will set in and she will be fine.
If however you are in a similar predicament with no dentist nearby, I would suggest that you 'store' the tooth till you reach the dentist.
Putting the tooth in water, or whatever liquid is available, is better than letting it dry out, but water can damage the cells of the tooth within minutes.
Milk is largely free from harmful bacteria, and the cells on the tooth are less likely to absorb milk and ultimately burst, which can happen far more quickly in water.
With proper care and quick action, the dentist or endodontist may be able to place the tooth in the socket so it can re-attach itself and function for many more years.
Source: Medicine Net