Heisenberg's uncertainty principle
On the very small scale when we are measuring something like an electron, however, each measurement does something to it. If we measure position first, then we change its momemtum in the process. If we measure the electron's momentum first, then we change its position in the process. Our hope would be to measure one of them and then measure the other before anything changed, but our measuring itself makes a change, and the best we can hope to do is to reduce to a minimum the energy we contribute to the electron by measuring it. That minimum amount of energy has the Planck constant as one of its factors.