The preposition per means across or through.

Jis eina per tiltą. He is walking across the bridge.
Tėvas bėga per gatvę. Father is running across the street.
Laivas plaukia per kanalą. The ship is swimming across the canal.
Asilas joja per upę. The donkey is riding across the river.
Paukštis skrenda per jūrą. The bird is flying across the sea.
Autobusas važiuoja per šaligatvį. The bus is driving across the pavement.