Fiction first. Ask questions and use the answers.

1. Ask what they're doing.

   I'm stabbing myself to I can heal someone.

2. Ask how that works in the fiction. If they come up with something that the group finds interesting and plausible in the fiction already established, let them do it. If not, say something along the lines of “It doesn't sound like your magic works that way.”

Fiction first. Ask questions and use the answers.

1. Ask what they're doing.

   I'm stabbing myself so I can heal someone.

2. Ask how that works in the fiction. If they come up with something that the group finds interesting and plausible in the fiction already established, let them do it. If not, say something along the lines of “It doesn't sound like your magic works that way.”