Late in the evening, the robozoom arrived at the robot princess's castle. The palace door was locked, and not a soul was in sight. He pressed the intercom button.
"Who dares disturb the sleep of the robot princess in the middle of the night?" a hoarse voice echoed from the loudspeaker.
"I am a robozoom, I have come from faraway lands on uncharted paths, fighting ferocious destroids along the way, only to be here now. I will remove the terrible curse from the robot princess, my future bride, and soon I will become the robot king myself," the robozoom replied pompously.
"Well, well... in that case," said the voice from the palace, "answer a riddle as your first test."

"I am round when I lie,
but I'm thin when the truth is mine.
What am I?"

The robozoom pondered, wondering what this could be, but nothing came to mind. The rhyme sounded familiar to him from somewhere, but he couldn't remember where. So he said something, hoping to guess correctly.
"I think it's the destroid! He's always lying, and he can't lose weight anyway."
"That's not true!" shouted the voice angrily from the intercom. "Because you are failed you will spend the night outside. The second test will follow in the morning. Oh, and I'd watch out for those wild destroids, because there are some lurking around the palace."
With that, the intercom went silent.