(The). Prop. 5, book i. of Euclid. This is the first difficult proposition in geometry, and stupid boys rarely get over it the first time without tripping.
It is the ass's pitfall, not his bridge.
If this be rightly called the “Bridge of Asses,”
He's not the fool who sticks, but he that passes.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894