As ink leaves the tip, new ink is drawn from the reservoir - which often consists of a large volume of a similar porous material to that used in the tip - by capillary action and gravity. As with a fountain pen, ink leaves the tip of a felt tip pen by capillary action when writing on a porous surface. However, unlike fountain pens, many markers can also reliably write on slick impermeable surfaces that are wet by the ink, and in such applications ink typically does not continually leave the pen as it is held against the writing surface.