I'm relaying this question from my son...
Tearing is a melacha. But, as far as I understand, one may tear open a food bag (like a bag of chips) on Shabbat. Yet, one may not cut between the letters that are on a cake on Shabbat, because that's considered erasing.
Why is tearing allowed but "erasing" not allowed?
I'm not limiting my question to these two comparisons. There are probably others, but I can't think of them.