What does the Heisenberg uncertainty principle state?
Heisenberg Uncertainty Principle - when we measure a particle, we can know it's position or its momentum, but not both.
The Heisenberg Uncertainty Principle starts with the idea that observing something changes what is being observed. Now this may sound like a bunch of nonsense - after all, when I observe a tree or a house or a planet, nothing changes in it. But when we are talking about very small things, such as atoms, protons, neutrons, electrons, and the like, then it very much does make sense.
When we observe something that is quite small, how do we observe it? With a microscope. And how does a microscope work? It shoots light down onto a thing, the light reflects back, and we see the image.
Now let's make what we are observing really small - smaller than an atom. It's so small we can't simply shoot light down at it because it's too small to see - so we use an electron microscope. The electron hits the object - say a proton - and bounces back. But the effect of the electron on the proton changes the proton. So when we measure one aspect of the proton, say it's position, the effect of the electron changes it's momentum. And if we were to measure the momentum, the position would change.
That is the Uncertainty Principle - that when we measure a particle, we can know it's position or its momentum, but not both.