Very complicatedly, I would say. Given the temperature and atmospheric pressure on Titan, both methane and ethane are at the triple point, and so goes between solid, liquid, and gas easily. This leads to a pretty complex chemical system and a cycle analogous to the hydrological cycle ("methane-ologic cycle"), the only other place in the Solar System. One of the famous features are that are seen to pop in and out of existence in the seas are the "Magic Islands", of which waves or bubbles might explain the transient nature.