Between Planets is a juvenile science fiction novel by American writer Robert A. Heinlein, originally serialized in Blue Book magazine in 1951 as "Planets in Combat". It was published in hardcover that year by Scribner's as part of the Heinlein juveniles.

Pay it forward is an expression for describing the beneficiary of a good deed repaying the kindness to others rather than paying it back to the original benefactor. It is also called serial reciprocity.