It's called "Forced Perspective"
Step 1. Go outdoors with a friend. Stand in front of a building.
Step 2. Take a picture so that the friend is really close to you and the building is really far away. Assure that the friend's head lines up with the top of the building.
Step 3. Ask your customer how tall it is, given just the photograph.
Do not specify what "it" is that the customer must define the height of. Let them assume -- or guess -- what part of the picture is relevant.