Slughorn explains, when everyone is assembled, that alchemy with an egg is actually where phoenixes are speculated to originate; but no one has the recipe any more and there haven't been new phoenixes for centuries. They will not be making living things today. But the eggshell is a good base from which to appear lots of possible objects; alchemy doesn't like to conjure things into existence in plain sight but is quite willing that new things appear inside eggshells, cocoons, geodes, and similar. Their eggs have been pre-drained of their interiors for this purpose, although some recipes use full eggs.
As last week, they are invited to come up with recipes on paper for review by the professor before trying anything. Known possible results available with this set of ingredients include wooden, decorated eggs nested inside the shell; a shellful of small sharp teeth (the commonest source of hens' teeth, as they have encountered once or twice in potions); successively smaller eggshells the smallest of which holds a blob of mercury; and a perfectly hardboiled egg which can be arbitrarily flavored by exact details of the process. But they are welcome to aim at another result as long as their procedure looks safe.
"I don't think there's actually a rule against it, just, you know, it'll break so why would you."
"I think Mr. Maxwell will probably have a better idea than us about what things are easy to observe and cheap enough that he doesn't mind owling them to Hogwarts."
"I suppose so. I'm wondering if small things like electronic watches are affected or if it's only more sophisticated tech."
"Well... yes, but there might be a limit to how much he wants to spend on miniaturized electronics for twelve-year-olds to take apart in the hopes that we produce adequate notes."
"Anyway, I'd rather let him design the experiment at least to start since he's a real scientist but as long as there's things we're curious about we may as well ask."
"Well, he says that the only difference between being a 'real scientist' and not is that real scientists need peer review."
"Tell him to include different types of gadgets. Like, calculators and electronic watches and torches and stuff."
"Maybe with the batteries sent separately in case that helps, so we can watch the whole fizzling process."
fog! Bright fog, with a sort of almost-shape, small and longish.
"Ooh!"
"Okay, lemme see, if I do concentrated stuff like last time... Expecto Patronum!" Now the shape is significantly more identifiable, and the blob is almost solid. Sadde squints at it. "Is that a weasel...?"