You've probably heard of the Scoville scale and know that it refers to the heat of a chilli but how is it actually measured? Named after its creator, Wilbur Scoville, the test takes an amount of chilli extract which is then diluted in sugar water until the heat is no longer detectable.
If something has no capsaicin (the bit in the chilli that's responsible for the searing heat), then its Scoville rating is 0. A Habanero, on the other hand, can have a rating as high as 300,000, which means it needs diluting 300,000 times over before no heat can be detected.
Even the same variety of chillie can vary wildly; it all depends on the plant's growing temperature, hours of sunlight, moisture, soil chemistry, as well as the kind (and amount) of fertilizer used. The infamous Dorset Naga (available from Peppers by Post), a relative of the Scotch Bonnet, boasts a Scoville rating of 923,000 units. Wow.