Illusion

This is the Ames Room.

This is precisely what you would see if you got two identical twins to stand in it and then you look into the room through a slit with one eye. Yes, the above is real and there was no photoshopping done.
What happens is that the room is constructed in this way:
Basically, the actual room itself is built in the shape above so that the corner where the "smaller" twin is standing is actually further than where the "larger" twin is. The tiles on the floor are also in actual fact specially shaped. The clock would also be actually much larger than it seemed to you. The whole thing is actually a play on light, perception and the brain's cognition. We bring the idea of depth and distance from real life into what we see. Like we see that the clock is of appropriate size to the person and we relate it to the distance that it looks to us and it seems reasonable. The floor tiles look proper too and not lopsided, so we assume that the room is normal. Amazing, right?

Here's another one called the Checkershadow Illusion:

Look at the squares labelled A and B. Are the squares really of a different shade?

Now look carefully.
The squares are actually of the same colour. Those who paint/shades regularly would know that this is one trick in Art.
What really happens is that firstly the checker patterns causes us to expect that B would naturally be lighter than A. The second reason has to do with the colours being in the shadow and in the light, but I'm not quite sure how to explain it. The amazing thing is that even though now you know that the two shades are really the same, the brain is already so... conditioned, that even if you look at the first picture again, you would still see the 2 colours as being distinctly different.