system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you block one end of a straw why does the liquid stay in place/in the straw?
gpt
There are three main forces at work here.

The first is a force called capillary action. In short, this is the tendency for a cohesive fluid (cohesive meaning it's sticky to itself and holds together, like water) to cling to the walls of the inside of a tube and generate an upwards force. This upward force combined with the waters surface tension, another product of waters cohesive properties, allow the fluid to stay intact inside of the straw instead of just running off and separating like oil would.

The second is gravity, which obviously pulls the fluid towards the ground

the third is suction, which causes the air pressure in the straw to drop as the water is pulled away by gravity. after a short drop, the pressure is low enough to provide a suction force powerful enough to suspend the fluid.

so basically, capillary action and cohesion keep the fluid together, and gravity combined with suction keep the fluid in place by pulling in opposite directions.