A ruler: the two lines are the same length.
Bigger or smaller?
Vertical–horizontal illusion
Which line is longer, the upright one or the flat one?
They are the same length. “Prove it” brings out a ruler.
Why? We overestimate up-and-down lengths, maybe because our eyes are side by side and our view is wider than it is tall. Tall towers and trees look taller than they are wide for the same reason.
Picture: S-kay, Public domain · Wikipedia(opens in a new tab)