Steam carries the heat away
Hot soup cools mostly from its surface. Water leaves as steam and takes heat with it. A bowl near 90C loses heat fast.
Fat works like a lid
Oil floats on water and does not turn to steam. A thin film cuts the steam off. The soup below stays hot much longer.
Fat also holds the smell
Many roasted smells dissolve in fat, not water. The fat layer keeps them close to your nose. Each sip smells like the first.
Shunta says:
Try two cups of hot water. Add a spoon of oil to one. Touch both after 10 minutes.
