system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a sail boat go back to shore where it came from if the wind is blowing against it?

Or further, how does a sailboat go east if the wind is blowing west?
gpt
Triangular sails are held at an angle to the wind. So if the wind is moving west, it might push the boat southwest. 

Good sails can make the boat go even faster than the wind at a right angle to the wind. So if the wind is blowing 20mph to the west, the boat might go 30mph due south. Think about stepping on an ice cube. It will squirt out to the side faster than the speed your foot was pushing down on it. You might say a boat traveling at a right angle to the wind is "squirting" out from in front of it. 

Really good sails and hull combinations can harness wind blowing at a small angle. So if the wind is blowing west, the boat travels southeast. A boat can never travel straight into the wind, but always has to at least sort of be traveling across it. 

So, to go east into a wind traveling west, first you go southeast, then northeast. You're always traveling across the wind. Switching back and forth like this is called "tacking".