"A very smart engineer I worked with marveled at how I can quickly do logarithms (for decibels) in my head."
A couple of days ago I was discussing with some friends how big an elephant could grow on the moon. (It makes sense in context). I said: well, gravity is six times less, assume the "design" of an elephant is the best it can get, weight=fn(mass), mass=fn(size^3), so we're looking at the cube root of 6, so.... one point.... eight, ish. We then giggled that "kids today" would consider us witches. "How did you do that?"
Well.... 2^3 is 8, so that's too big, so let's start with 1.8*1.8*1.8, call it 18 so 18*18, call it 20, so 20*20 is 400, that's too big, so we need to take two 20s off, so 360, and we need to take two more 18s off, call it another two 20s, so that's 320, remembering that'll be a bit too small, so now times another 18, call it 20, so 640, that's too big, take two 20s off, 600, so around about 6.0, so the cube root of 6 is around about 1.8, so an elephant could be about 80% bigger on the moon than the earth.
Witchcraft!
calc.exe tells me 1.8^3 is 5.8, and Google tells me root(6,3) is 1.82ish, so good enough for government work.
These days, that would barely cover the cost of fuel to get to the office. 
