The observation that, because of the very large number of degrees of freedom in an unfolded polypeptide chain, the molecule has an astronomically large number of possible conformations, and therefore sequentially sampling all the possible conformations is not practical, yet most small proteins fold spontaneously on a millisecond or even microsecond timescale.

The situation in which a phenomenon being observed for study is unwittingly influenced by the presence of the observer.

plural of twin paradox

A choice problem designed to show an inconsistency of actual observed choices with the predictions of expected utility theory.

A paradox of choice in which people's decisions produce inconsistencies with subjective expected utility theory.

(linguistics) The apparent paradox that children learning English encounter many sentences amenable to dative shift (e.g. "give the book to me" → "give me the book") but apparently have no way to learn that this is not possible with certain verbs (e.g. "*donate me the book" is unacceptable), and yet rarely make this kind of error.

(biology) The apparent paradox that additive genetic variation is maintained despite persistent female choice for particular male traits.

The phenomenon of an arrow travelling in the direction where it points when the bow is fully drawn, when it seems that the arrow would have to pass through its original position. The explanation relates to the bending of the arrow when released.

(set theory) A demonstration of a surprising property of infinite sets. Some positive integers are squares while others are not; therefore, all the numbers, including both squares and non-squares, must be more numerous than just the squares; yet for every square there is exactly one positive number that is its square root, and for every number there is exactly one square; hence, there cannot be more of one than of the other.

(fluid dynamics) The contradiction that, for incompressible and inviscid potential flow, the drag force is zero on a body moving with constant velocity relative to the fluid.