That makes it sound like a straightforward classical effect. It's weirder than that. In the two-slit experiment, if you measure which slit a photon (or other small particle) passes through, then after a bunch of particles you get two spots on the wall. If you don't measure, you get an interference pattern, as if the particle were a wave that passed through both slits.
This is true even if you make the decision about whether to measure, after the particle has already passed through the slit. Check the measurement after the particle passes the slit, but before it hits the wall, and the interference pattern goes away.