kotrljati

1. (+ acc.) to roll
   Deca kotrljaju loptu po travi. — The kids are rolling the ball across the grass.
   Radnici su kotrljali bure niz rampu. — The workers were rolling the barrel down the ramp.
2. (kotrljati se) to roll
   Kamenje se kotrljalo niz padinu. — Rocks were rolling down the slope.
   Suze su joj se kotrljale niz lice. — Tears were rolling down her face.

Without se it means "to roll something"; with se, "to roll" by itself.