system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what's it called when water runs down the side of the cup while you're pouring it and how does it happen?
gpt
It has to do with surface tension. If you fill a glass up all the way and then add a tiny bit more, you will see the water form a little dome rather than spilling. Water will do its best to not break apart under stress. 

Completely falling away from the rest of the water in the cup when you're pouring it is something it would rather avoid since it would need to break the surface tension entirely and form a completely new drop, but if it can flow down the side of the glass, it doesn't need to worry about the surface tension for the part of the stream that's still touching the glass...but then gravity takes over since there's no longer anything beneath it and that causes the water to break away from the cup anyway. Since it already was sticking to the side of the glass for a while, it's flowing in a weird direction now and momentum (plus a little more surface tension) will make it go sideways for a while instead of straight down.