Maggie's Farm

A useful term - a noun - for "a problem that severely tests the ability of an inexperienced person." More generally, a problem or challenge which will separate the bright and the perceptive from the not-so-bright and the not-so-perceptive.

"Bridge of asses." Donkeys do not like to cross bridges.

In mathematics, the term is applied to the problem from the first book of Euclid that if two sides of a triangle are equal then the angles opposite those sides are also equal.

Traditionally, the bridge of asses referred to Euclid's Fifth Theorem of planar geometry, the comprehension of which and the implications of which were and are a sticking point for less-bright students.

By the way, this is a good if somewhat challenging book: Experiencing Geometry. A bit of a pons asinorum itself.