Does it really need to explain this one? LOL
The Joke:
I can tell if someone is lying just by looking at them.
Well...I can also tell if they’re standing.
2 meanings of Lying:
Lying can mean not telling the truth.
Lying can also mean being in a horizontal position (not standing).
So when the person says “I can tell if someone is lying just by looking at them,” it sounds like they mean detecting when someone is not telling the truth.
But the punchline reveals they actually mean physically lying down, because they add:
“Well… I can also tell if they’re standing.”