Why do people buy water after eating pretzels?

The salt on the pretzels, as was mentioned, causes thirst. This is because the the body attempts to normalize the amount of salt concentration throughout the body through osmosis. When salt goes into you, waters transfers out of the cells, trying to even out the salt concentration inside and outside of the cells. This makes you feel thirsty. For more information, look up the topic of osmosis.

Note: I am a mathematician, not a Biologist, so this may be incorrect.