Believe it or not, this is one of the classic pranks.
(Earliest written account dates to ancient Rome, but I'd bet the Sumerians pulled it off too!)
It seems that a cow will readily climb up a steep slope, such as a flight of stairs, but will refuse to climb down.
(Don't ask me why, I don't know, I'd say ask a cow, but "MOO" doesn't explain a lot.)