Gamp's Law of Elemental Transfiguration

Gamp's Law of Elemental Transfiguration is a law of wizarding physics. There are five Principal Exceptions to Gamp's Law; the only one explicitly stated is food - however, there are a number of possible candidates for the remaining four.

It should be noted that whilst food cannot be outright created from nothing, it can be multiplied if one already has some food to multiply (such as through the Engorgement Charm) or summoned if one's known the rough location and is fairly sure the food will still be there.

Behind the scenes

 * Gamp's Law apparently uses the surname of the person who discovered it. Hesper Gamp, a witch married to Sirius Black II, might be that person or a relative of that person.
 * Food cannot be created, but this is what Minerva McGonagall seems to do in Harry Potter and the Chamber of Secrets. However, it is most likely that she used a spell that can summon food from one place to another without travelling through the middling space. This may resemble the charm or spell used by the Hogwarts house-elves, which transfers food from one plate in the kitchens to its counterpart above in the Great Hall. Molly Weasley also appears to create a creamy sauce into a pan in Harry Potter and the Goblet of Fire. However, Gamp's law announces that you cannot create new food out of nothing, but you can conjure it if you know where it is. You can also increase the amount or quantity of food, which is what Harry Potter does in Harry Potter and the Half-Blood Prince when he performs a Refilling Charm on several bottles of wine.
 * Food cannot be created; but since animals can be conjured (such as birds, using the Avis spell; snakes, using the Serpensortia spell; or buffaloes, as Filius Flitwick, in Harry Potter and the Philosopher's Stone, cites Baruffio as having done by accident) or transfigured (such as when McGonagall transformed her desk into a pig), one may theoretically take meat from the animal, which would supposedly allow the [indirect] conjuration of food. However, there may be some unforeseen side-effect of attempting this which preserves this rule (such as the constituents of the meat vanishing as quickly as they were conjure following ingestion).
 * According to W.O.M.B.A.T., it may or may not be possible to turn an animal into a human. It also may be possible to make the inedible edible and not all objects can be transfigured into food.

Possible principal exceptions
Other possible candidates for the four remaining principal exceptions proposed are: love, (pre-existing) life, information and precious metals with the reasoning for each explained below.
 * Love - known to be impossible to simply create in the Harry Potter universe, it can only be simulated through a Love Potion, Entrancing Enchantments or the Imperius Curse, although even in these cases the best one can hope for is obsessive infatuation that fades when the spell or potion fades.
 * Life - whilst it is show throughout the series that new life can be created (e.g. birds produced using with Avis and flowers with Orchideous) it is also firmly known that life which has already ended is impossible to be restored (which would include via conjuration). Hence, pre-existing life is impossible to conjure and therefore may be one of the exceptions to Gamp's Law. Interestingly, though, pre-existing life - like food - can also be multiplied (in size, at least) and summoned (such as with the Resurrection Stone).
 * Information - a logical assumption, or else one would assume that many things that were seen as impossible in the series would be achievable; for example, students might conjure all necessary information before a test or Aurors might conjure information pertaining to the whereabouts of a wanted criminal (like Voldemort or Sirius Black)
 * Precious Metals - if wizards were able to conjure precious metals and therefore money then their economy would collapse or at the very least there would be no impoverished wizarding families (like the Weasleys), however, this is clearly not the case. Though one might counter this with the fact that J. K. Rowling once said "There is legislation about what you can conjure and what you can't" and that money may therefore be one of these substances whose creation is illegal, this does not seem to be the case, which leads one to think it was never banned because it cannot be achieved. For example, if the creation of gold was banned then one would expect Nicolas Flamel to have been arrested long ago and yet was never. The Philosopher's Stone is also reminiscent of the exception to the rule relating to food in that it only transforms other metals into gold but cannot create it (like how food can be transformed but not created). Likewise, Leprechauns may create objects that look like gold or money, but which will, in time, change back to dead leaves or dirt; it is merely an illusion or transfiguration (once again similar to the allowance of transformation but not creation). Similarly, the Geminio Curse allows one to multiply (but not conjure from nothing) the desired object (in this case something made from gold), as was witnessed in the Lestrange Vault at Gringotts in 1998, which reminds one of the exception to the food rule allowing multiplication but not outright creation.

Appearances

 * Harry Potter and the Deathly Hallows