A leak is a hole or other opening, usually unintened and therefore undesired, in a container or fluid-containing system, such as a tank or a ship's hull, through which the contents of the container can escape or outside matter can enter the container. The word "leak" is also used as a verb; matter going through the opening is said to leak. The entry, exit, or exchange of matter through the leak is called leakage, the subject of another disambiguation article. The matter leaking in or out can be gas, liquid, a highly viscous paste, or even a solid such as a powdered or granular solid or other solid particles. A leak can be between two (or more) fluid-containing systems, allowing transfer of matter from either system to the other, or even an exchange of matter between them. Leakage of matter into a container or other system could be called inleakage. Leakage of matter out of a container or other system could be called outleakage. The presence of a leak does not necessarily mean there is always leakage of matter; it just implies there could be leakage through the opening. If the container or system is empty in an empty environment, there is no leakage at that time. Sometimes the word "leak" is used to refer to leakage in a certain situation, for example the passing or making public of secret information.