Setting: In Angel's apartment at some point between "Parting Gifts" and "Five by Five". Probably closer to the latter.
Inspiration: I had a theory and I thought it made an amusing premise for a one-shot. (And then Angel brooded all over it and made it less amusing, but I still love him.)
Disclaimer: *resigned sigh* They're not mine.
"I notice that you never heat your blood in the microwave," said Wesley thoughtfully as Angel filled a large mug with pig's blood from one of the containers in the fridge. "Is that because it would rupture the red blood cells and render it useless?"
Angel stopped pouring and stared across the kitchen at Wesley in mild bewilderment. "Uh, no," he said, replacing the container in the fridge and closing the door. "I just haven't figured out how the microwave works yet. I've never had one before. Usually I just drink it cold."
"Ah," said Wesley after swallowing a large bite of the scrambled eggs on toast Angel had made for him and Cordelia (who had yet to arrive). He then picked up his glass of orange juice and drank a few mouthfuls.
Angel frowned and looked at the mug of blood in his hand as he sat down at the table opposite Wesley. "Is that really what would happen if I used the microwave?" he said, feeling disappointed. His body generated no heat. Far from being refreshing, therefore, cold drinks were horribly chilling and tended to make him a little sluggish for a while. He only drank his blood cold because he normally didn't have time to heat it, but just because he was used to it didn't mean he liked it. He had been looking forward to figuring out the microwave so he could heat up his blood in a matter of seconds.
"I expect so," said Wesley, his brow furrowed. "Microwave ovens work by exciting the water molecules in the food, which causes them to vibrate more quickly and heat the surrounding molecules through friction. It wouldn't take long to destroy blood cells that way, considering their high water content."
Oh, fun, thought Angel. Modern science. Science had never really appealed to him the way languages, art, martial arts, and literature did—at least not the types that didn't have some sort of practical use in his daily unlife, so he hadn't really bothered to keep up with it over the years. He was beginning to suspect that this had been unwise. It would have been very useful, for instance, to have known more about the human respiratory system back when Buffy needed CPR. Willow had explained it to him afterward, and he didn't like the idea that he could have been the one to revive her all along but hadn't known it. What if Xander hadn't been there?
"Would that really make the blood useless?" Angel asked, shuddering slightly as he shook off the awful implications of that last thought.
"Well," said Wesley, swallowing his current bite of toast and putting his fork down, "nobody is exactly certain what it is that vampires require from the blood they drink, but it seems only logical that the structural integrity would be important."
"Yeah, I guess," said Angel.
"Obviously, most vampires drink it directly from the still-living source, ensuring that the blood is warm, fresh, and alive, so that would never be an issue for them. You, on the other hand, have had experience with, erm, 'leftover' blood. Do you find that it's less, er, satisfying the less fresh it is?"
Angel normally would have felt uncomfortable discussing the finer points of his diet with any human, but Wesley had that intense look he always got when getting carried away with a line of inquiry—a look of earnest fascination that was devoid of judgment or condemnation, and this put him at ease. "That could explain why it takes longer for me to heal if what I'm drinking is more than a few days old," he said with a shrug.
"Exactly!" said Wesley enthusiastically. "The older the blood is, the greater the number of blood cells that have died, which presumably means it becomes progressively less beneficial to a vampire over time. Which would be the same principle that applies with normal blood transfusions, after all. Dead blood is of no more use to living humans than it is to vampires."
Angel mulled this over while he drank his blood, allowing Wesley time to finish his breakfast and revel in the afterglow of new discovery.
Early Wes amuses me to no end. Also, if you want to read more about Willow explaining the respiratory system to Angel, see chapter six of "Beneath the Surface".