Ah, the double slit experiment. A classic in the world of quantum mechanics. Basically, it involves firing particles (say, electrons) through two slits and observing the pattern that they create on a screen behind the slits. The weird thing is, even when the electrons are fired one at a time, they still behave as if they are interfering with each other - creating an interference pattern on the screen. This suggests that the particles are behaving like waves, rather than discrete particles. It's a mind-bending experiment that challenges our understanding of reality itself.