What body part is the metric system based upon?
legs
arms
feet
fingers

i guess that visual aids present persuasive examples to what you are presenting, some factors would be theme (does it fit with the color, information, what you are presenting), size (can you see it, is it a big enough span of data/information), and overall look of the visual aid.

hope this : )

ice to water 1 water to vapour 2 latent heat