Three Skullcaps (D5)

After being captured by the tribe of the forest, the three explorers were taken to the tribe's chief, who declared that the tribe needed a successor of great wisdom. He showed the captives five skullcaps, three of which were red and the remaining two were green. He commanded the three to line up with their faces toward the wall. On each he placed a red skullcap.

"Now", the chief said, "turn around and look at each other. Whoever can tell me, with a logical explanation, the colour of the skullcap you are wearing will be granted freedom."

The three turned around, looked at each other, and after a long pause one of them said, "I don't know." Two natives impaled him on their spears.

After another pause, the second prisoner stuttered, "I don't know." The two natives threw him into a fire pit.

Immediately, the third man turned a cartwheel, announced, "My skullcap is red," and proceeded to explain.

Amidst the applause of the gathered tribesmen, the chief awarded the explorer vice-chiefdom of the tribe for his wisdom

How did the explorer know that his skullcap was red?