vaporo

The word the compounding article is built on. If vaporo feels familiar before you have met it, that is because vaporŝipo, a steamship, is the worked example the course uses to teach how Esperanto glues roots together. Vaporo steam plus ŝipo a ship gives a kind of ship, because the last root is the head and everything to its left narrows it down. Turn the pieces round and ŝipvaporo would be a kind of steam. That page also makes the point that compounding is what you reach for when the affixes run out: there is no suffix meaning powered by steam, so the long way round is the only way.

And it is the word behind the hyphen rule too. The orthography article uses ŝarĝvaporŝipo, a cargo steamship, to show when a compound needs a hyphen: nothing in the grammar tells you whether to read it as ŝarĝ-vaporŝipo or ŝarĝvapor-ŝipo, and writing the hyphen decides it for the reader. Three roots is where that starts to bite.

The fence with the other things in the air. Vaporo is water in its gaseous state, the stuff over a kettle and on a cold window. Fumo is smoke, which comes off a fire and is not water. Nebulo is fog or mist, water hanging in the air in droplets rather than as gas. Nubo is a cloud. English lets steam and vapour and mist slide about; here they are four separate roots and swapping them reads as an error.

The family. Vaporigi is to vaporise something, vaporiĝi is to evaporate by itself, and the pair is another clean demonstration of the -ig- and -iĝ- contrast this band keeps running into. Vaporŝipo a steamship, vaporlokomotivo a steam engine, vaporbanejo a steam bath, vapormaŝino the steam engine that started all of it.