"A summon spirit is..." She stopped. This would require a more in-depth explanation than usual. "A summon spirit is a being representative of mana, the life force of our world. They're like...guardians, I guess. In order to be able to summon, the summoner - that's me - has to make a pact with each spirit, by stating a vow. If I break the vow, my pact with that spirit is ended, and someone else is free to make a new pact with them."

She stopped for a second. Hopefully Anita was following this. "It takes a lot of mana and strength to summon. Usually I can only do it once, and with a lot of preparation beforehand. ...Every once in a while the situation is desperate enough that I can just call them, though."