A container with a varying capacity is provided. The light guide plate 16 is a plate-like member and includes a light entrance surface 16c through which light enters and a light exit surface 16a through which the light in the light guide plate 16 from the light entrance surface 16c exits. A seal circumferentially surrounds the outer surface of the wall and is spaced from the peak of the barb. The mask may also include a gas exit port having a valve operatively connected thereto, so that gas flow may be restricted when pressure within the mask and carbon-carbon composite brake disc falls below a specified minimum value.