Economical with the truth

Imagine you asked your friend if they ate the last cookie, and they said, "I didn't eat the whole thing." They didn't say they didn't eat any of it, but they also didn't tell the whole truth. They were being economical with the truth. It means they are only telling part of the truth and not being completely honest. In the grown-up world, people might do this to make themselves look better or avoid getting in trouble. But it's always better to be honest and tell the whole truth, even if it's not easy.