Why one needs to be specific when explaining things to a child

Ken Burns has produced a new series about national parks.

We were discussing the series this weekend and sharing stories of family vacations at various national parks. One friend remembered her mother telling her, before a childhood vacation started, that they were going to Mt. Rushmore where they would get to see the heads of four famous presidents.

This is what she actually saw:

But this is what she imagined she was going to see: