Lubricating grease is unique in that, in general, it stays where it is placed in the application, such as a bearing. It does not normally leak out of poorly sealed applications like liquid lubricants do. In addition, grease forms a seal to keep out contaminants like dust, dirt, water, and corrosive gases. It acts as a carrier for lubricating solids such as graphite, molybdenum disulfide, or PTFE. There is some evidence that grease provides extra film thickness in lubricated contacts beyond that which would be provided by a liquid lubricant of similar viscosity grade. Grease also provides for cost effective maintenance, since grease relubrication intervals can be quite long.