I have been reading/learning about the double slit experiment, its implications in quantum theory, and how it explained that 'particles' can behave as both waves and particles. I know that the wave function is a probability of the location of the particle, and that shooting the electrons through the double slits causes an interference pattern associated with multiple waves. This, though not making intuitive sense (in relation to how anything can even exist as a wave), is something I can follow. However, I have read/heard that an 'observer' collapses the wave function into a single point. This is what caused the electrons to actually show up on the wall behind the slit; however Feynman (admittedly, as a thought experiment) suggested that putting an 'observer' prior to the slits would cause the electrons to fly through as particles, and leave no interference pattern on the back wall. What is an 'observer'? How and why would they electron 'know' it is being observed and therefore cause it to change behavior?