A time travel paradox in which the consequences of an event in the future travel back in time and cause an event in the past or present, which in turn is among the causes of the first event, forming a causal loop in which each event is caused by the other.

The proposition in hydrostatics that any quantity of water, however small, may be made to counterbalance any weight, however great; or the law of the equality of pressure of fluids in all directions.

Removal of one's clothes while in a state of hypothermia.

The paradox of time travel in which inconsistencies emerge through changing the past.

The observation that the association of two variables for one subset of a population may be similar to the association of those variables in another subset, but different from the association of the variables in the total population.

The perceived failure of European countries to translate scientific advances into marketable innovations.

(astronomy) The observation that the night sky is mostly dark, yet, in a boundless universe of stars, every line of sight from the eye must eventually intercept the surface of a star.

(relativity) The phenomenon in which two observers who start together with identical clocks, follow different timelines, and then rejoin can have different elapsed times on their clocks, especially if one travels at a relativistic speed.

(relativity) The clock paradox.

(topology) The non-intuitive fact that it is possible to turn a sphere inside out in a three-dimensional space with possible self-intersections but without creating any crease.