(mathematics) A theorem in set-theoretic geometry, which states that given a solid ball in three‑dimensional space, there exists a decomposition of the ball into a finite number of disjoint subsets, which can then be put back together in a different way to yield two identical copies of the original ball.

A paradox of deontic logic, whereby the plausible statements (i) if you murder, you ought to murder gently, (ii) you do commit murder, and (iii) to murder gently you must murder imply the less plausible statement (iv) you ought to murder.

A paradox about a person's expectations about the timing of a future event which they are told will occur at an unexpected time. It is based on the idea that a prisoner, told that he will be hanged on a day unknown to him, concludes that "I cannot be hanged on Friday because if I haven't been hanged by the end of Thursday then it would no longer be a surprise", and so on backwards through the week.

(genetics) The observation that the C-value of an organism is not dependent on its complexity (presumed number of genes)

An apparent paradox in probability theory, illustrated by various questions about the probability of a child within a family being of a particular sex.

(economics) The paradox raised by a particular (theoretical) lottery game that leads to a random variable with infinite expected value (i.e. infinite expected payoff) but nevertheless seems to be worth only a very small amount to the participants.

plural of St. Petersburg paradox

The phenomenon that people can sometimes recover more quickly from more intense emotions or pain than from less distressing experiences, tentatively attributed to the fact that only the more intense states trigger psychological defense processes.

A hypothetical problem dealing with the notion of infinity. Given an empty vase and an infinite supply of balls, an infinite number of steps are performed, such that at each step 10 balls are added to the vase and one ball removed from it. The question is then posed: how many balls are in the vase when the task is finished?

A philosophical problem related to Benardete's paradox, but involving countably many Grim Reapers, each of which will kill the person in question at a certain time (if still alive), taking a certain period of time to do it.