That's interesting, I'd never thought of visualizing a thousand of something that way - I'd have thought using tiny indistinguishable units that cover each other up is admitting defeat from the get-go. I've always thought in terms of those little centimeter blocks they have to visualize powers of ten in grade school. A thousand is simply a 10x10x10cm cube (a cubic litre), quite easy to visualize. There are a also a thousand millimeters in a meter, and a thousand people is also a common order of magnitude for a group of people in a school context (population of the school, or a single year of students, or the number of people attending a sports game, depending on the size of the school).