These words all refer to the action that a liquid or gas undergoes in passing through another substance or medium. To filter means to pass (a liquid, air, etc.) through a porous substance such as paper or cloth, in order to extract certain impurities or essences. Papers folded into the shapes of cones and inserted in the mouths of test tubes or coffee-making devices are familiar uses of filters . Photographically, certain kinds of light or certain colours are filtered by mean so specifically coloured and constructed glass or plastic lens coverings.

Percolate means to pass or cause to pass through fine opening or interstices. In coffee-making, as the hot water passes through a container of finely ground coffee, it extracts the essence of coffee and carries it along with it. Percolate emphasizes the process of passing through or permeating a substance; filter points to a particular means by which this process can take place.

• Rainwater percolates through the soil to form an underground reservoir; To get red, you have to filter out the greens and blues; The spilt ink filtered through the blotter.

Exude and seep mean to ooze or trickle out through pores or small openings. Seep suggests a gradual and often accidental or unwanted movement of a substance into or out of something.

• Gas seeped into the room; Air seeped from the balloon; Blood seeped from his wound and reddened his bandages.

Exude is often used of natural or planned phenomena: to exude sweat; Gum exuded from the tree. It is also used figuratively with the sense of a powerful diffusion of any quality: to exude charm.