The professor waved his arms enthusiastically and told us to stop thinking. “Orders of magnitude only!” he explained. “Let’s call it 1 cm/year. What about rates of mountain uplift in active regions, like the Himalayas and New Zealand?” The answer, to an order of magnitude, was 0.1 cm/year.
Now his point was obvious: plates drift faster than mountains rise, but both move at similar speeds. This relationship – normally hidden behind numbers like 7.3 and 0.4 – was suddenly clear in the comparison of 1 and 0.1. We had lost some precision, but had gained clarity, which we would easily remember.