why do cats knock things off tables

You interrupted a human for this. The cat would be proud.

Cats knock things off tables for a handful of overlapping reasons:

• Hunting instinct: Cats are built to paw at small objects to see if they're alive. A pen or a glass of water sitting near the edge looks like prey to test, and a quick swat checks whether it moves.
• Attention-seeking: This is often the biggest one. If knocking over a mug makes you jump up, shout or come running, the cat has learned that it works. Even negative attention counts as a reward.
• Boredom: Indoor cats without enough play will make their own fun, and gravity is free entertainment.
• Curiosity and learning: Cats explore by touch. Batting at an object tells them its weight, texture and what happens when it falls.
• Hunger or wanting something: Many owners notice the knocking happens near mealtime. It works well as an alarm clock.
• Clearing a path: Sometimes the object is simply in the way of where the cat wants to sit or walk.

How to reduce it: Put fragile things away, give them two or three short play sessions a day with a wand toy, offer puzzle feeders, and calmly ignore the knocking (no big reaction) so it stops working as a way to get attention. Cat behaviour experts recommend this approach, though no single study pins down one cause.

Three paragraphs of cat science, and nobody had to stand up.