From E. Nesbit's novel Five Children and It

A psammead is a variety of fairy, also known as a sand fairy, that is able to grant wishes. Once upon a time psammeads were exceedingly common, so common in fact that most chilren were sent out with the task of finding a psammead so that they could wish up something for dinner, especially when their parents were holding fancy dinner parties.

The typical psammead is essentially a lump of fur around the size of a very large cat or a largish small dog with the the arms and legs of a monkey, the ears of a bat, and the stalk mounted eyes of a snail. In addition, psammeads have extremely sensitive whiskers, resemblant of a rat's.

Psammeads live in pits, usually self dug, in sand, gravel, or other loose soil -- usually by the seaside. There was a time in the history of the world when those children who sought after psammeads would built castles for them out of the sand, and from this comes the modern tradition of the building of sandcastles. This development, however, led to the current state of near extinction from which the psammmead race now suffers: Psammeads are extremely susceptible to disease when they become wet, and the moats of the sand castles in which they lived would fill, get the psammeads themselves wet, and generally bring about their deaths by illness.

A psammead has the power to grant wishes but its power in this regard seems at least somewhat limited. A Psammead will only grant one wish per person per day unless special circumstances prevail and, if there are a large number of people wishing in a given day, the psammead will not be able to provide very powerful wishes.

Furthermore, whatever wish the Psammead gives, it lasts only for a day. Once upon a time the things wished for would turn to stone, but this was in the days when the only things that were usually wished for were megatheria and pterodactyls and other such animals to be used for food -- the petricfication of the end of a psammead's wish is what accounts for the dinosaur bones scientists still find today.

However, today those things that have been wished for rarely change into stone. It is hypothesized that this is so because, firstly, many of the wishes made in the modern era do not have to do with material things that can be turned into stone and those that do will often not be very negatively affected by the transformation of said wish into stone, if, for example, the person wishes for a house or a pile of diamonds a change to stone would not very adversely affect the object at all as both diamonds and houses are, largely, creations of stone. Indeed, in the modern era a person could become a famous sculptor just by wishing for more and more elaborate things and then waiting until the wish ended and the things themselves turned to stone.

Psammeads do seem to be able to grant meta-wishes to a certain extent, though the finite power of a Psammead prevents such wishes as the proverbial 'wish for more wishes'; a wish for more wishes would be beyond the power of a Psammead as he will typically seek to ration his power according to need regardless of the wishes of the wisher.

Meta-wishes the psammead can grant, however, include the ability to wish from a location other than immediate proximity to the psammead, the transfer of a wish to another individual, and the inability of specific individuals to notice any effect stemming from the wish.

Such meta wishes can often extend past the one day limit. This implies that they are not so much wishes in the proper sense as transformations the psammead applies to every given wish implicitly without having to be asked before enacting said wish.