Hey kid,

Let me tell you about the ‘special theory of relativity’ that Einstein wrote about. I will try and explain in the simplest possible terms for a 11 year old kid.

1. We believe what we see (and if we hear anything, which we are not used to seeing, we find it difficult to understand and believe). In our world, even the fastest things move very slowly compared to the speed of light (e.g. earth goes around the sun at a very fast 30km/sec but is still slower than light by a factor of 10,000). However, our world view would be very different and weird / unbelievable if we would see something moving at or close to the speed of light.

1. In our world, we believe that
1. Speeds always add up i.e. if you run towards me at 5 m/s while I stand, that is what your speed will be to me. If however, I too start running towards you at say, 10 m/s, to me, you are now getting closer to me at 15 m/s. Our speeds have added up.
2. Size of any object is the same irrespective of who sees them. If you hold a 1 m long stick in your hand, I will agree to its length even as u run towards me.
3. Time moves at the same rate for all. As u run towards me, both of us will agree that our wristwatches are ticking at the same speed.
4. If I see two events (say two balloons popping) happen simultaneously, you will agree with me (that they happened simultaneously) even as u run to me.

1. However, in reality, things will change dramatically if you increase your speed and start running towards me at a speed equal to or close to the speed of light.
1. The speed with which you move towards me will NOT add up beyond a point. For e.g. if I too start running towards you at the speed of light, we can still only move towards each other at a max speed of light. We can’t run towards each other any faster. And no, this is not an illusion. The speed of light can never be exceeded – this is an absolute truth.
2. We will measure size of objects differently. To me, the stick in your hand (which you claim to be 1 m long) will be smaller to me. A 1 m long stick in my hand will be smaller to you too. Both of us would be right! No illusion again. Distance (length) is not absolute and depends on the observer. Yes, it is weird!.
3. To me time will go slower for you (as per what I can see on your wristwatch) than for me (as per my wristwatch). In effect, from my point of view, you will age more slowly than me! You will say the same to me! Both of us would be right again! Time again is not absolute, how fast it moves again depends on the observer. Yes, this too is very confusing. But take Einstein’s word for it.
4. When I will see two balloons popping near me simultaneously say 1 km from each other, you will disagree with me. To you, neither did they pop simultaneously, nor were they 1 km away from each other. You will claim that the balloon closer to you popped later. Both of us would be right! No, I am not kidding.

1. There are some interesting paradoxes which will happen. Two of the famous ones are
1. Twin paradox – if you fly away from me at the speed of light, turn back and fly back at the speed of light, I would have aged many more years than you when we meet again. Yes, it would actually be true! And no, the vice versa is not true, i.e. I will not be older than you from your point of view. (why is our situations not identical? Because you did the turning thing, not me. More on that when u grow up!). No illusions.
2. Pole in the barn paradox – say you are driving (speeds close to that of light) a train that you want to hide in a tunnel. I am standing next to the tunnel. As per you the length of your train is 200 m, while as per me the length of the tunnel is 150 m. If you speed is sufficiently high, you will claim that your train is too long to ever be able to hide completely inside the tunnel (remember the tunnel will be even smaller to you than it is to me, similar to the stick example). However, I will claim that your train will be able to fit inside the tunnel completely. Both of us will be right!

Now for one clarification. The effects described above don’t just take effect at speeds equal to or near to speed of light. They hold true even in our real day world; even when a car moves at say 50 km/hour or even when you walk slowly. Its just that at such speeds these effects are too tiny and hence not noticeable. So, remember this; if you see me walking down the road, I am a bit thinner (just a very very tiny bit) to you than I am to me and I also age a bit slowly (again, just a tiny tiny bit) to you than you do to yourself. And I look at you and its the same for me. Lengths measured and times elapsed are relative & depend on how one moves relative to another.

To summarize, reality as per Einstein is almost completely opposite to what we have believed since the day we were born. Speeds cannot add up indefinitely (not beyond the speed of light), distance and time are not absolute truths but depends on who is observing them. Simultaneous events for one may not be simultaneous for another person.

Don’t worry if it all appears confusing at first or even forever. We are in the same boat. Except if you are running towards me at a speed close to that of light, then…..Both of us would be right!

ps: If this piqued your interest, kid, I suggest u wait for a few years and then take this beautiful course (which is where I learned a bit about special theory of relativity). https://www.coursera.org/learn/einstein-relativity taught by Professor  Larry Randles Lagerstrom at Standford University.