change the filter

A lot of machines use "filters".

For example, an air conditioners has a filter to clean dust out of the air. A car engine has a filter to clean dirt out of the oil. Water purifiers have a filter that cleans the water.

After you use a filter for a while, it gets dirty and stops working as well. When that happens, someone has to "change" the filter. That means replacing the old one with a new one.

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