What is it that makes something 'real'?

This was a question I, and a few classmates of mine, asked one of our teachers when I was in grade school. This particular teacher was pretty well known as being incredibly smart, so we were hoping he could provide us an answer. What he told us is that other than gravity, there's nothing that proves that anything is 'real', not even other people. He said we have no idea if other people see the world like we do, or if they can even see the world at all. He said that there's nothing, besides gravity, that we can all relate to and if we were to assume that other people were real, we should assume that they too, see and experience gravity.