How do video games cause bad grades

Video don't cause bad games directly. The TV does. Have you ever heard that TV turns your brain to mush. That because when you watch TV for a long time, chemical changes happen in the brain, making it feel like "mush". You probably play video games a lot and this can happen. It kinda messes you up for a while and you wont feel quite right. Limit your time on your TV and study, and your grades should be fine.

Do good in school! :)