Guess the riddle

I start with an easy one and I'll increase the difficulty. Those who guess it we'll have a lollipop. The finalist (the one who guesses more riddles) by the end of the term will have a super prize!

PD: (For Marina and for all) this is for everybody including students that are now in batxillerat ;-P

What occurs once in a minute, twice in a moment, but never in an hour?