What is the greenhouse effect ? (in your own words)

Have you ever been inside a greenhouse? Greenhouses are very warm inside all year round — this is how plants are grown inside them, even in the winter. The way a greenhouse works is that the glass they are built from allows the sun's rays to shine in, but then prevents the heat from escaping once it is inside.

Now, think of the Earth as being a giant greenhouse. The gasses on Earth act just like the glass; this is how the Earth gets warm from the sun even though it is about 93 million miles away. The gasses allow the sun's rays to shine in, but then prevent the heat from escaping the Earth. This way of warming the Earth's surface is referred to as the greenhouse effect.

(Source: https://www.enwin.com/kids/conservation/greenhouse_effect.cfm)

when it is at an intermediate population density