What is a mirage?
We see things because our eyes can sense light, and our brains can figure out what that light means and tell us what we are seeing. Light usually travels in a straight line, bouncing from the object that we see to our eyes, but sometimes, light can change course.
A mirage is an optical illusion.
That means something is playing a trick on our eyes. In the case of mirages, that something is temperature! In cartoons, sometimes you'll see characters lost in a desert. They see what they think is an oasis (a pool of water,) and they'll run to it thinking they can get something to drink. When they get there, the oasis vanishes! It was a mirage! That's the idea many people have of mirages and it's based on truth.
Desert sand is very hot, and it heats the air right above it. As the warm air rises in the sky, it quickly cools, and it actually becomes thicker! It might be hard to imagine air being thick, but air is actually made of a lot of stuff: elements like nitrogen, carbon, and the oxygen we need to breathe. In warm air, all the air particles are spread out, but in cool air, they are actually packed together more tightly. When light hits the thick air, it can actually bend!
On a hot desert day, something that you see on the ground in front of you might not actually be there. It could be light coming from somewhere else, like the sky. What would the sky look like if it were on part of the ground? It would probably look like a lake or an oasis. That's just one example of a mirage.
You might see light being bent above hot pavement on a summer day. The air might seem to blur things. It's actually just a mirage!
Other things besides different temperatures of air can bend and stretch waves of light. One of those things is water.
If you have a clear bottle filled with liquid and you look at your fingers through the bottle, what happens to them?
If you have a glass of water and you dip your finger in it and look at your finger through the glass, does it look different? Is it bigger? Smaller? Stretched? Squished? Maybe it's in a slightly different location than it should be.
What is the water doing to the light that bounces off your fingers?