Why are igloos warm?

It is a very good question. The secret is the snow itself. It is not solid ice, but snow that is hard-packed. This snow has many small pockets of air inside. This trapped air is a fantastic insulator. It works a bit like the insulation in a house wall.

The heat comes from the people inside. Their body heat warms the air. The snow walls stop this warm air from escaping. The igloo's shape is also very clever. The entrance is low to keep cold air out. People often sleep on a raised platform. This is where the warmer air collects. So, it can be -40 degrees outside, but inside it is much more comfortable, sometimes above freezing.