maybe you only think
it's a bad idea until you actually hit yourself over the head with a blunt object, only after doing it do you know
this is an example of a posteriori
knowledge - based on empirical evidence - you performed the experiment (hitting yourself), and the resulting pain convinced you it wasn't a good idea.
before the experiment, it was a belief, which turned into a truth, and knowledge came out of the intersection. it may have been a truth before the experiment, but you didn't know
however, there's also a proiri
knowledge, which is stuff you already know before and without experimenting - eg i know that 'all bachelors are unmarried' - i don't have to get off the couch and talk to every single bachelor to find out whether or not he is married. i know already beforehand, without needing an experiment. Einstein seems to ignore this sort of knowledge.
then there's more abstract stuff - i 'know' about imaginary numbers, for example, but never experienced them - or do i only 'think' that i 'know'?
it's certainly a head-scratcher that has puzzled philosophers for thousands of years, and has puzzled mickey for decades.
if you like scratching your head, read about epistemology.