Well, what do you think? There's only space for 70 in there, so when you add the 70th item (indexes from 0, remember), where does it go?

The answer is, most likely on top of whatever's defined next. This can cause all sorts of amusing bugs.
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Ohh, you're right, of course it would cause a problem. However, in my homework we don't have to worry about it so I just counted the numbers manually Remember, we're just starting to cover Arrays...