muelejo

Decoding it, in two pieces. Mueli is the verb: to grind, to mill. -Ej- is the suffix that names the place where the action happens - the one you already know from lernejo a school, panejo a bakery, loĝejo a flat. So a muelejo is a grinding place, which is a mill. Note the shape of the compound: the whole point of -ej- is that it attaches to whatever verb you like, and a learner who knows the suffix never has to be taught the building.

A word about mueli itself. It has no page in this course, because it does not reach the top three thousand roots on its own - the building outranks the verb, which happens whenever an activity is more often named than done. Learn it here anyway, because it is the base of the whole family: muelilo is a grinder or a mill in the sense of the machine (kafmuelilo, a coffee grinder), muelisto is the miller and arrives at rank 2,755, and muelŝtono is the millstone. Mueli is also freely figurative: to grind an argument to dust.

What was ground, and by what. The input is greno at rank 2,164, the output is faruno at 2,403, and the power came from either the vento or the rivero - ventmuelejo and akvomuelejo, both transparent, both built from roots you have had since the first thousand.