The same thing as knowing how delicious a certain food is.
Sure, but then there is no problem in knowing, ever. You said "we don't know, but we can make an estimate" and with respect to my personal opinion about how delicious a certain food is, I have immediate direct knowledge and no need for estimates.
I think it's past time for another Stupid Questions thread, so here we go.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Please respect people trying to fix any ignorance they might have, rather than mocking that ignorance.