A pie is a baked food, with a baked shell usually made of pastry dough that covers or completely contains a filling of fruit, meat, fish, vegetables, cheeses, creams, chocolate, custards, nuts, or other sweet or savoury ingredients. Pies can be either filled, where a dish is covered by pastry and the filling is placed on top of that, top-crust, where the filling is placed in a dish and covered with a pastry/potato mash top before baking, or two-crust, with the filling completely enclosed in the pastry shell. Some pies have only a bottom crust, generally if they have a sweet filling that does not require cooking. These bottom-crust-only pies may be known as tarts or tartlets. An example of a bottom-crust-only pie that is savoury rather than sweet is a quiche. Tarte Tatin is a one-crust fruit pie that is served upside-down, with the crust underneath. Blind-baking is used to develop a crust's crispiness, and keep it from becoming soggy under the burden of a very liquid filling. If the crust of the pie requires much more cooking than the chosen filling, it may also be blind-baked before the filling is added and then only briefly cooked or refrigerated. Pie fillings range in size from tiny bite-size party pies or small tartlets, to single-serve pies and larger pies baked in a dish and eaten by the slice. The type of pastry used depends on the filling. It may be either a butter-rich flaky or puff pastry, a sturdy shortcrust pastry, or, in the case of savoury pies, a hot water crust pastry.