Episode 4 - "Biyomon Gets Firepower"

As the group re-enter the forest, they see a black gear flying above. They soon reach a desert with standing utility poles, and Mimi's compass becomes ineffective due to the presence of metal in the sand. They discover a village of Yokomon in the middle of the desert, and Sora's partner Biyomon is questioned as to how it was able to evolve from Yokomon. They are taken to a nearby water spring but discover that the spring and its source aquifer have dried up and are ablaze with fire. Meramon, a fire Digimon enraged by the black gear that entered its body, descends from Miharashi Mountain at the edge of the desert and heads toward the village. As the group evacuate the Yokomon residents, Biyomon challenges Meramon but is no match for its power. Biyomon evolves into Birdramon and attacks Meramon from the air, releasing the black gear from inside its body. As a sign of gratitude for saving their village, the Yokomon invite the group for dinner, but the children discover they cannot eat the food.

Extracted from:  http://en.wikipedia.org/wiki/List_of_Digimon_Adventure_episodes (1st July 2013)