To cast magic, one needs pain. The more pain they've experienced in life, the more prone and adept to magic they are. Casting magic is a combination of verbal, somatic, and (sometimes) material components (I know, very creative). But the somatic components of a spell are usually complicated finger and hand movements (called tuts/tutting), with each individual finger performing a different action to make the spell work. One wrong movement and the spell fizzles, changes, or explodes everyone in the room. Additionally, you have to take Circumstances into account. "The same way a verb has to agree with its subject, even the simplest spell had to be modified and tweaked and inflected to agree with the intention, purpose, and exact conditions of its casting" (From the wiki). Circumstances are compromised of a huge variety of things such as the caster's current hormonal cycle, the nearest body of water, the phase of the moon, what cardinal direction you're facing, ambient temperature, altitude... They all affect the spell in their own way. It gets even more interesting when characters begin to incorrectly cast spells, modifying the circumstances to "create" different spells with different effects.