I like to think of the bike problem. Sit down and draw a picture of a bike from memory in as much detail as you can in 2 minutes. Easy right? But it is surprisingly hard.
I tend to think in abstractions so to me it's a triangle (frame), 2 circles (wheels), a T shape (seat), an inverted L (handlebars), a letter 8/infinity (bike chain)...
Maybe I'm just weird :)
Maybe Ascii art simplified things :-). I think what tends to trip people up are things like - Where do the pedals actually live in relation to the rest of the frame? How/where does the seat attach? The front wheel? Most people know there is kind of an angled bar on the frame, but where does it go exactly?