Otoshi buta are Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

ese-style drop-lid

Lid

LID is an abbreviation for:* Local light injection and detection, a method to determine fibre splice losses, especially in polarisation-maintaining fibre splicing...

s for use in Japanese cooking. These round lids float on top of the liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 in a pot while simmering

Simmering

Simmering is a food preparation technique in which foods are cooked in hot liquids kept at or just below the boiling point of water , but higher than poaching temperature...

 foods. They ensure that the heat is evenly distributed and reduce the tendency of liquid to boil with large bubbles. This reduces the mechanical stress on the food and keeps fragile ingredients in their original shape.

Otoshi buta are almost always made from wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, so they have to be soaked in water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 for a few minutes before use to avoid absorbing the flavor of the dish and disturbing the flavor of the next dish cooked using the otoshi buta. After use, the otoshi buta is washed and dried completely before storing.

Though it depends on the recipe

Recipe

A recipe is a set of instructions that describe how to prepare or make something, especially a culinary dish.-Components:Modern culinary recipes normally consist of several components*The name of the dish...

 and food

Food

Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

 being cooked, otoshi buta may be substituted with a sheet of tough cooking paper or aluminum foil.