What falls as rain on Titan?

Methane. At around minus 180 degrees Celsius, methane sits near its triple point on Titan the way water does here - so it exists as gas, liquid and solid, forms clouds, rains, collects into rivers and pools into lakes and seas. Titan is the only place besides Earth with standing liquid on its surface and an active weather cycle.

The rain is strange to picture. Titan's gravity is about a seventh of ours and its atmosphere is dense, so the drops are large, up to about a centimetre across, and fall slowly - a lazy drizzle you could probably watch descend. Rainfall is rare and torrential rather than regular, and the surface shows dry channels and evidence of flash flooding.

The Huygens probe landed in 2005 and photographed a shoreline covered in rounded pebbles of water ice - rounded, which means something has been tumbling them in a flow. At that temperature water ice is rock, and methane is the liquid that erodes it. Titan therefore runs a complete hydrological cycle with the roles of the materials swapped, which is the closest thing to an alternative version of Earth's plumbing anyone has found.