Question

I don't understand why when you use a straw the higher pressure outside of it pushes the water up the straw.

Answer 1

pressure is force acting on an area .
in the same area, more pressure means more force.

2 person, pressure outside and one in straw push water against each other. more pressure outside, so more force it pushes ... where will the water flow?

Answer 2

When material stuff is free to move, it moves away from stronger force on it to weaker force on it. OK ? Now take it to the next step: When material stuff is free to flow ... like liquid or gas ... it flows from higher pressure to lower pressure. That's what makes wind, and that's why water comes out of a garden hose when you open the nozzle. It doesn't amaze you at all when you INCREASE the pressure at one end of the straw (blow into it), and air moves DOWN and out of it. So it shouldn't bother you that when you DECREASE the pressure at your end, anything that's free to flow UP the straw will do that.