IN: The return address is not required on postal mail. However, lack of a return address prevents the postal service from being able to return the item if it proves undeliverable; such as from damage, postage due, or invalid destination. Such mail may otherwise become dead letter mail.
does there have to be a return address on an envelope?

OUT: no

IN: The accident came to be known as the ''Miracle on the Hudson'', and a National Transportation Safety Board official described it as ''the most successful ditching in aviation history.'' The Board rejected the notion that the pilot could have avoided ditching by returning to LaGuardia or diverting to nearby Teterboro Airport.
could flight 1549 have made it to an airport?

OUT: no

IN: Rotation in mathematics is a concept originating in geometry. Any rotation is a motion of a certain space that preserves at least one point. It can describe, for example, the motion of a rigid body around a fixed point. A rotation is different from other types of motions: translations, which have no fixed points, and (hyperplane) reflections, each of them having an entire (n 1)-dimensional flat of fixed points in a n-dimensional space. A clockwise rotation is a negative magnitude so a counterclockwise turn has a positive magnitude.
are there any fixed points under a rotation?

OUT: yes

IN: The term conservative force comes from the fact that when a conservative force exists, it conserves mechanical energy. The most familiar conservative forces are gravity, the electric force (in a time-independent magnetic field, see Faraday's law), and spring force.
is the force of a spring a conservative force?

OUT:
yes