How does my tablet know when I open the cover? I've recently bought a magnetic cover for my tablet (a bamboo smart cover for Asus Transformer Pad TF303CL). What puzzles me is that whenever I open the cover, the screen turns on. How does the tablet know when I open the cover?

The cover isn't connected in any way to the tablet, AFAIK. The only way that I could think of is if the tablet is using the light sensor, but then why doesn't it turn on the screen when I use my finger to cover the sensor? Or when I turn the light off and then on again?
Your device will have a little magnetic sensor hidden under the display that can trigger the display to turn on.

A small magnet lies embedded directly under the surface of the smart cover and lines up with the corresponding spot on the device. The device detects the removal of this magnet when opening the cover and turns the screen on, and can also put the device into sleep mode when the cover closes.