(graph theory) The problem of finding the shortest closed path or circuit that visits every edge of a (connected) undirected graph.

(mathematics) A mathematical problem where one must determine how many grains of wheat are on a chessboard, assuming one grain of wheat is placed on the first square, and each subsequent square having twice as many grains of wheat as the previous square.

(economics) The situation where those who benefit from goods or resources do not pay for them, resulting in a lack of funding and subsequent underprovision.

A conspiracy theory postulated by David Icke in which the government (or another higher power) manipulates the population by introducing a problem and then using their own means to solve that problem.

(mathematics) The problem of predicting the motion of a group of celestial objects that interact with each other gravitationally.

(philosophy) The problem, considered by Ludwig Wittgenstein (1889-1951) and other philosophers, of whether it is possible for a private language to exist.

A geometric puzzle involving calculating the width of an alley based on the height at which two ladders cross when propped diagonally across it.

The problem of finding the motion, relative to one another, of two objects, where the position of each affects the motion of the other.

(computing theory) A generalization of the shortest path problem to graphs that are only partially observable (i.e. the graph is revealed while it is being explored).

(computing) An issue for computing and data storage situations in which time values are stored or calculated as a signed 32-bit integer, and this number is interpreted as the number of seconds since 00:00:00 UTC on 1 January 1970. Such implementations cannot encode times after 03:14:07 UTC on 19 January 2038