The more you know about a quantum particle's position, the less you know about its momentum, and vice versa. This is the Heisenberg uncertainty principle, and I thought it applied to quantum particles only, so I didn't mind not understanding why this was so because quantum physics is weird. But 3Blue1Brown says the Heisenberg uncertainty principle applies to non-quantum things to, and in this 20-minute video, he explains why. You can actually get a reasonably good understanding of the uncertainty principle by watching the first two minutes of the video.
