Tell me how you did it, and you'd better teach me. Thank you teachers.

Divide 10 bottle of water into two parts, each with 5 bottles, and then put it on the balance scale to weigh ①.

Take five bottles of water (no salt water) from the lighter side. On the heavier side, take out four bottles of water from the remaining five bottles and put them on both sides of the balance in pairs.

If it is balanced, then the remaining bottle is salt water. But if it is unbalanced, then the salt water is on the heavier side, then put these two bottles on both sides of the balance respectively, and call them ③.

The heavier one is salt water.

You have to weigh it at least three times to ensure that you find this bottle of salt water.