When steam is released into a cylinder, it expands into the space available. If a piston is placed inside the cylinder, the pressure of the steam and its expansion will push on the piston. When the piston reaches the end of its stroke, steam is admitted to the cylinder on the other side of the piston. This pushes the piston back. The steam used for the initial stroke is now pushed out of the cylinder as the piston returns and is exhausted into the smokebox, where it escapes through the chimney into atmosphere. The puffs of exhaust steam escaping into the air make up the characteristic sound of the steam locomotive. In detail, the cycle is as follows. Before the piston starts to move, steam is admitted into the space between the cylinder end and the piston face to build up pressure. This is known as 'lead'. Once a certain amount of steam is admitted into the cylinder and the piston starts to move, the supply of steam is cut off. Now, natural expansion of the steam takes place and the piston pushes as far as it can go. For the last 25% or so of its stroke, the exhaust is opened and steam starts to escape. By the time the piston starts its return stroke, the same process is being repeated at the other end of the cylinder. Ahead of the piston during its return stroke, steam from the previous cycle is exhausted until it is 75% along its stroke, when the port is closed and the remaining steam is compressed up to perhaps 30% of the normal admission pressure. At this point the lead position is reached again and the whole cycle is repeated.