This is probably the only BtVS / Twilight fanfic I'll ever write, prompted by a thread on the Twisting the Hellmouth forums. All characters belong to the usual suspects, this isn't an attempt to infringe on their copyright etc.

Basic Physics
Marcus L. Rowland

“I’m sorry,” said Edward, “I knew that my form of vampirism had a price, but I never realised...”

“You never spent much time with humans until me,” Bella said quietly, “how could you know?”

“Basic physics, love,” said Spike. “Vamps like me, we’re powered by blood and magic. Vamps like him, non-magical, don’t drink much blood... well... the energy has to come from somewhere. Didn’t you wonder why he sparkles?”

“I just thought it was the sun.” Bella coughed, and Edward wiped away some blood.

“Well, the sun’s nuclear, I suppose, you were half right... Bloody emo radioactive sparklepires...”

end.

No, I'm not going to explain the physics...