Your son is too young for these explanations. Just tell him: "You're crazy!" then slap him and tell him to shut up.

This answer has been selected and edited by New Scientist staffWhen my son (now nearly 10) was a similar age, I tried to explain this phenomenon during a train journey. First I pointed out that objects further away look smaller. I used his hands to show this: if he held one hand close to his face and the other at arm's length, the one at arm's length appeared smaller, even though he could put his hands together to confirm they were the same size.Secondly, I showed him that it takes more objects to fill the same amount of visual space if they are further away. For example, if the hand further away is half the apparent width of the one closer, it takes two hands to fill the same width.Finally, I got him to think about something moving, such as an index finger traced slowly from one side of his palm to the other. If it moved at the same speed when it was further away, it travelled the same actual distance (a palm's width), but seemed to have travelled only half as far. So it would take twice as long for it to look like it had travelled the same distance. I then summed up by explaining that the distant things were not actually moving slower, they just looked as if they were.I also had to explain why it looked as if the trees and houses were moving when my son was sure that they weren't really. First, I got him to move his hand in front of his face, and then to hold his hand still but move his head from side to side. In each case he could see that the hand seemed to move across his vision in the same way. I told him that the two movements were equivalent and he seemed to accept that.The other passengers on the train thought I was a little strange, but it kept my son quiet.Keri Harthoorn, Stoke-on-Trent, Staffordshire, UK

This answer has been selected and edited by New Scientist staffThe answer is that the type of optical system that is used by our eyes causes us to perceive a particular object as "smaller" the more distant it is - a phenomenon called foreshortening. As our vision system converts the angles subtended by the things we are looking at into apparent distances on our retina, this causes nearby objects to sweep through our field of vision much more rapidly than distant ones. So while distant and nearby objects are within the same field of vision, those further away take longer to pass across it, as they have a low angular velocity, than those that are closer.You can demonstrate this by placing your hand on a newspaper. Make a "V" with your index and middle fingers and sweep it along the text. Your hand is the car, and the V is your field of view. You can see that the text near your fingernails takes a long time to move from one finger to the next, while the text closer to your hand moves more rapidly.Gregg Favalora, Arlington, Massachusetts, US

This answer has been selected and edited by New Scientist staffOne way to demonstrate this process is to put a toy car on a path representing the road, with an object placed 30 centimetres ahead and 30 centimetres to the side of it. Show your son how the object goes from being diagonally ahead to diagonally behind the toy car when you move it forward 60 centimetres. Then do the same thing, but with the object 3 metres ahead and 3 metres to the side. This time the car has to travel 6 metres to cause the same change in the angle at which someone in the car would view it. Also point out that it takes much longer for the car to travel 6 metres as for it to travel only 60 centimetres.Eric Kvaalen, Cambridge, Massachusetts, US

Perhaps an age-appropriate answer would be most suitable for your four-year-old. I'm not sure what that would be but when our family dog was a puppy, she used to stick her head out the window on car trips and watch the shadow of the car as it whizzed past the ground. She was fascinated by it. I remember doing the exact same thing as a child. As she became an adult dog she stopped this and took more notice of the passing scenery and not the shadows---as did I. There must be a developmental neurological mechanism of perception in most mammals that makes them notice certain things at certain ages. You might try having your child look at the scenery through a cardboard tube that rolls of paper towels are wrapped around. Through that narrow view the scenery all seems to go past at the same speed.