Magic usually needs to be internally self-consistent, at least.
It's not like you have to explain how it works from a scientific perspective, but you should from the perspective of a mage or someone knowledgeable about magic.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather