system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people tap their glass on the bar before taking a shot?
gpt
The cheers and clinking of glasses together is well documented as having come from medieval times and the sign of trust that your drinking partner didn't slip you deadly poison. But what about the tapping of the glass after the cheers? The reasons for this are simple...

Firstly beer, wine, and liquor, i.e. distilled spirits, were safer to drink in "olden times" than water for purity reasons. We also know that the common man, soldiers and knights, and other lower nobility drank beer because wine and liquor were expensive. Also, back then, drinking was some of the only entertainment to be had and so games and thus traditions that have been carried on to even today, were started by people who drank beer.

So what is the fundamental, and provable, point of the "tap"?

1. After you "clink" your mugs/glasses/pints/etc you cause the head, or the foam, to rise. Tapping it on the bottom causes the head to fall so that you can chug/swill the beer with less of a problem, especially in a drinking contest setting.

2. If you are in a drinking contest the sound of your cup hitting the table, before you drink, and not when you start drinking is usually what the "judges" go by, as to when you started. That is also why the cup is slammed down afterward, to signify the end of your drinking "round".

Where did this tradition come from? Well probably sometime early on when beer became extremely popular as a drink for the masses. Sometimes these obscure traditions and games grew slowly over time and were altered from generation to generation and spread around until we have what we have today. It's like asking about the invention of the game of "Quarters"... but we'll get to that some other time.