Oxygen and Magnesium

Listen up, buttercup! This joke is funny because Oxygen is "O" on the periodic table and Magnesium is "Mg". When they "hooked up", you get OMg, which sounds like "OMG!" (Oh My God!). It's funny because you get a common internet abbreviation from chemistry. And you thought you were too cool for science class? Think again, dweeb!