A lubricant is a slippery substance that reduces friction between surfaces that rub against each other. When you oil a squeaky door hinge, you're using a lubricant to help the metal parts slide smoothly instead of grinding and sticking. Without lubricants, moving parts would wear down quickly from all that rubbing, generating heat and eventually breaking.

Car engines need oil as a lubricant because thousands of metal parts are constantly moving against each other at high speeds. Without oil, the engine would overheat and seize up in minutes. Bicycle chains need lubricant too, which is why they're often coated with grease or oil.

Even your body produces natural lubricants. Tears lubricate your eyes so your eyelids can blink smoothly. Saliva lubricates your mouth so you can swallow and speak easily. Athletes sometimes apply petroleum jelly to prevent chafing where their skin or clothing rubs during long runs or swims.

The most common lubricants are oils and greases, but some materials like graphite powder, or even soap and water, can work as lubricants in different situations.