i have periods where i am very busy, so sorry for how late this is.
Now this is about "youngs double slit experiment." This is an extremely well documented and repeated experiment.
Now for anyone thats doesnt know what this experiment is about you can find a link here (which was linked to me when asked this question.)
I find that this explains it much better than i couple through writing.
Anyway, so i was asked why observing the electron causes it to change what it does and what observation means in this case.
So what we have to understand is that an electron and other small particles on this scale do not behave as we would consider normal. They are not waves and they are not particles, they are in effect both at the same time. This might seem strange and confusing but im barely scratching the ice how complicated this can get.
The fact is that these particles are in fact a superposition of states. This means that there are many many states this particle can be in each, are possible but have varying degrees of likeliness (probability changes). So when we say it moves as a wave it does but the wave also corresponds to the probability of its states.
Looking at that video we can see that there is constructive and destructive interference, this applies to the probability. so with constructive interference there is a higher probability of the particle being there and lower for destructive.
So we have a collection of states moving and behaving like a wave. This creates what is known as a "wavefunction". This wavefunction describes this particle. However, if we do something to this particle for example observe the particle then the wavefunction collapses and a value is produced eg its position. Once the wavefunction is collapsed we know where its going and its velocity etc. In other words, it acts like a particle, not a wave.
I speak like this is all true but quantum mechanics are still heavy disputed so take it all with a pinch of salt.
The idea is that this wave of probabilities moves as a wave. There is no specific point that this electron or photon is positioned. It moves as a probability of its existence being in different places. However once we measure it, it exists in a spot. We have removed all the other probabilities of its existence and we know for sure where it is. It cannot therefore move as a wave anymore.
So momentum and position measurements do this to a particle. however every single form of observing a particle will give us an idea of its momentum or position. eg light uses photons and photos interact with particles and the wave they interact gives us information. So the fact of the universe is that we cant "see" the particle in anyway without collapsing its wavefunction and defining its position or momentum.