Dihydrogen monoxide (also known as DHMO) is a type of joke that makes fun of how people can panic about something that is actually very safe and harmless. It's made up of two hydrogen atoms (H) and one oxygen atom (O), and it's the same thing as water. Water is something we all rely on for our daily needs, like drinking and showering and agriculture. But when you call it dihydrogen monoxide, it sounds scary! That's why people joke about it—it's funny because it's ridiculous to be afraid of something so necessary and safe.