Why it is hot when i chew green chilly.?

Answer:
The substance which makes food hot is capsaicin. Capsaicin is the main ingredient of pepper spray.
How hot food is, is measured by the Scoville units (SHU), this is a measure of the concentration of capsaicin in food. Jalapenos chilli peppers contain 3000 to 6000 SHU. The hottest chilli pepper known - the Dorset Naga pepper - contains 970 000 SHU and is not really fit for human consumption.

---

Chillies have got a pungent and deep chemical called capsaicin, It activates the nerve cells on the tongue so that the brain experiences the sensation of heat. Rest of the stuff is the reaction of the brain.

---