Sari was introducing Bumblebee to the joys of shopping. The yellow mech hadn't quite got the hang of the human currency, and it didn't help that his fingers were too big to handle coins and banknotes. So Sari was responsible for looking after the cash. It made her feel terribly important. Her father had always been generous when it came to giving her money, but she appreciated it a lot more when she had someone to spend it with.
Something had been bugging her, though.
Sari and Bumblebee stopped to look at the dispay in the window of a department store. Sari quietly cleared her throat.
"So, what do you use to pay for stuff?" she asked. "Back on your planet, I mean."
Bee seemed more interested in the plasma screen TVs for sale. "Energon."
Sari gave him an odd look. She was a bright kid, and she'd been talking to Ratchet. There were still a few things that she just didn't quite get about Autobot physiology, though. "Is that the same stuff as the purple crud that comes out of you when you're hurt?" she replied.
"Pretty much."
"So, you use your own blood as money?"
Bumblebee thought about it. "Actually, it's more like... Well, I guess the closest equivalent you humans have is food."
Sari's eyes widened. "So let me get this straight. You bleed energon. And you use it to pay for things. And you also eat it."
Bumblebee shrugged, and gave her a blank look. "Yeah, I guess."
Sari levelled an accusing finger at Bumblebee, and and squealed at him:
"ROBOT VAMPIRE."
And that was why Bumblebee wore a Dracula costume on Halloween.