In double slit experiment of electron, we see an interference pattern when we don’t know through which slit electron has passed and we don’t see an interference pattern when we know through which slit electron has passed. Now, to identify the slit through which electron has passed, we use photons. We see at slits using photons and this act destroys the interference pattern. We say that interaction of photon with electron is destroying the interference pattern. But photons are always colliding (and hence interacting) with electrons while it is passing through the slits. So, how come this interaction affects the interference pattern only when we catch the photon i.e. only when we observe the electron and not when photon is simply colliding with electron and we don’t catch that photon?