mrviti

1. (+ acc.) to crumble
   Baka mrvi hleb golubovima u parku. — Grandma crumbles bread for the pigeons in the park.
2. (mrviti se) to crumble
   Ovaj kolač se mnogo mrvi kad ga sečeš. — This cake crumbles a lot when you cut it.

The perfective is izmrviti or smrviti.