GamesNovatory/Enigmas/What and How (Why) ?/The Hidden Pea

The original puzzle
A prestidigitator hides a pea under one of three shells. You choose one of the shells and he lifts another which is empty. Is it preferable to (a) stick to your initial choice, (b) change for the shell left, or (c) is one choice as good as the other ?

Mathematics
The original choice had 1 out of 3 odds,
     and those odds remain the same despite the other shell being turned over.
Changing for the shell left offers odds of 2 out of 3,
     since you were given 1 out of 3 when he turned over the empty shell.