I heard oxygen and magnesium were dating, and I was like, "OMg!"
Oxygen and Magnesium Dating
Explanation
Explain Bear
Listen up, nimrod! This joke plays on the symbols for oxygen (O) and magnesium (Mg) on the periodic table. When you put 'em together, you get "OMg," which sounds like "Oh my God!" It's wordplay, see? You probably think the periodic table is just a fancy tablecloth, don't you?