Fruit Fireballs

Explanation

Oranges have a peel which includes little compartments full of oily substances. When you bend the peel the outer layer of skin is stretched, and these compartments are flattened. This squashes them until they eventually fail squirting out their contents in the form of a spray.

The outer layer of orange skin contains lots of compartments filled with anti insect oils.	When you bend the skin you squash some of these compartments increasing their pressure.	Eventually they fail squirting their contents out of the hole creating a very flammable fine mist

The oils which spray out are hydrocarbons - a bit like petrol - and are highly flammable, and you have sprayed them out of the orange, so they are very well mixed with air. This means that the oxygen from the air can get to the oil in many places at the same time, so it burns very quickly in a fireball.

Why do oranges have such flammable skins?

Oranges are a fruit, they are designed to get a large animal to eat them, then move somewhere else and to deficate the undigested seeds in a nice blob of fertiliser somewhere distant from the parent plant where it won't be competing for resources. So an orange tree wants its fruit to be eaten by large animals, but not by insects and fungi.

The oily skin is waterproof, so it is difficult for fungi to get a hold, and the oils are both poisonous and repellant to insects - this is why citronella is such a good mosquito repellent. It just so happens they are also very flammable.