The Joke:
Why should you never trust stairs? They’re always up to something.
It’s a play on words using the phrase "up to something."
- Meaning 1: Stairs literally lead up to another floor.
- Meaning 2: If someone is "up to something," it means they are being sneaky or suspicious.
The joke pretends stairs are "sneaky" because they are always "going up." (Up to something)