1. A sarcastic way of letting a person know that whatever they are doing at a particular moment in time isn't working for them.
2. (rare) A genuine way of letting a person know that they are highly skilled or proficient in a certain area. Almost impossible to differentiate this one from definition "1"
1. (Drunk girl is dancing funky, as if having convulsions, at a club. Walk up to her and tap her on the shoulder): "Wow! Can you teach a class on dancing??"
2."How do you do it, man? You're a true BLG. Can you teach a class??"
A term usually used as a put down to those who ask obvious questions and have their questions answered in a sarcastic way. It resembles the classic burned or owned, but more specific to question-asking. In forums "teach!" is often followed by an emoticon wearing a graduation cap and holding a pointer.