How in tarnation does the photon know it's being watched and act the opposite way? Kind of mindboggling when you think about it?
Do you realize the distance between the nucleus of an atom and the electron is an enourmous distance. An atom is mostly empty space.
Get small. Climb inside a vaccum cleaner...;-)
For example, when electrons are fired at the target screen in bursts, it is easy to account for the interference pattern that results by assuming that electrons that travel in pairs are interfering with each other because they arrive at the screen at the same time, but when laboratory apparatus was developed that could reliably fire single electrons at the screen, the emergence of an interference pattern suggested that each electron was interfering with itself; and, therefore, in some sense the electron had to be going through both slits. For something that most people continue to imagine to be an unimaginably small particle to be able to interfere with itself would suggest that this "sub-atomic particle" was in two places at once, but that idea is strongly at odds with the truism, "You cannot be in two places at the same time."
Maybe it's the same way that you know that someone in the car in the next lane is looking at you. :-)