Dihydrogen monoxide (DHMO) is a combination of 2 hydrogen and 1 oxygen (di = 2, mon = 1), which creates H2O. It is a completely safe element, but a hoax that was created in the 1990's as an April Fools joke got popular that said that DHMO was a chemical that was a substance that was contained in things such as acid rain, nuclear power plants, etc.
John: "Have you heard about the chemical Dihydrogen monoxide? I read that it was really dangerous and it's even in our water and food!"
Steve: "No, that is just water. Do more research and don't believe everything you read online."