There are lots (an infinitude) of smooth functions that coincide with f(n)=n!$f(n)=n!$ on the integers. Is there a simple reason why Euler's Gamma function $\Gamma (z) = \int_0^\infty t^{z-1} e^{-t} dt$ is the "best"? In particular, I'm looking for reasons that I can explain to first-year calculus students.
Fixed a typo in the definition of the Gamma function