The Magma Mephit has a neat little ability called Change Shape:
Change Shape (Su) Once per hour, a magma mephit can assume the form of a pool of lava 3 feet in diameter and 6 inches deep. The mephit’s damage reduction improves to 20/magic when in this form. The mephit can’t attack while in lava form but can use other spell-like abilities. It can move at a speed of 10 feet, but it can’t run. In this form the mephit can pass through small holes or narrow openings, even mere cracks. The pool’s touch ignites flammable materials such as paper, straw, or dry wood.
Since Su abilities default to a standard action, we can forgive that the ability has no listed action cost.
But how on Oerth does it turn back into a mephit? Does it do this at-will, or does its magma form expire somehow?