What is a greenhouse? How does air inside it remain warm?

Hint: A greenhouse is used in locations where the weather and temperatures are very cold to grow tropical plants that require a lot of heat, sunlight and warmth to grow. Due to green houses some important plants can be grown anywhere in the world.

Complete step by step solution:
A green house is a building that is made of glass inside which plants can be grown all year round and even in areas where the climate is cold and not optimal for the growth of tropical plants which require a lot of heat and sunlight.
A greenhouse keeps the air inside it warm. This is because the glass traps the sunlight or solar radiation by letting the solar rays enter but does not let it escape. Hence, the heat energy from the sun gets trapped inside and becomes concentrated. This leads to a rise in the temperature of the air inside the green house.
This provides the required heat and light to the plants. Therefore, even in places where the temperatures are low and not a lot of sunlight is available, with the help of greenhouse gases, plants can be grown. Greenhouse gases also make it possible to grow summer plants even in winters.

Note: A similar effect is shown by the greenhouse gases (carbon dioxide, water vapor, etc.) in our atmosphere. They let the heat from the sun enter into our atmosphere but do not let it escape after it gets reflected from the surface of the earth. Hence, they help in keeping the planet warm and make it possible for life to be possible. However, too much of these gases is causing a problem in the current scenario as they are not letting the heat escape and the earth is getting warmer and warmer. This is causing global warming, which is a major environmental concern.