Monday Puzzle

The Presurfer, in cooperation with pzzlr.com, brings you a puzzle every Monday. Just to tickle your brain.
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Liam would like to take an apple to school to give to his teacher as a gift. There are 8 apples in his mother's fruit bowl from which to choose. LIam knows that all the apples weigh the same except one which is heavier. He would really like to give his teacher the heaviest apple but the problem is that they all look pretty much the same.
Luckily he has a balancing scale but is running out of time as the school bus is due at his door any minute. What is the minimum number of weighings necessary for Liam to be sure about which apple is the heavier one?
You can find the answer here.