People make things for money. Things like clothes and tables and cars. When people work to make these things, they also make other stuff like smoke. When people drive cars they make a different kind of smoke. All of this smoke makes the air heavy and thick. This thick heavy air is like a warm cover and like the covers on your bed, it makes the world hotter and hotter.
At the top and bottom of the world there is a lot of ice. There is so much ice that if the world gets too warm (because the whole world is getting hotter) and the ice becomes water, it will not fit. Water will leave its bed and cover the land where cities are and people live. Water will cover the land where food grows. This is already happening. The top of the world is too warm and the water is already going up.