skii

Two vowels in a row and both of them pronounced: ski-i, two syllables, and the same again in skiis and skias. Esperanto has no silent letters and no diphthong here, so the doubled i looks stranger than it sounds. It belongs to the small set of verbs whose root ends in the same vowel as an ending, which the language handles by simply writing both.

The noun is skio, a ski, one of the boards. Skioj in the plural is the pair, skiado is skiing as an activity, and skianto is the person doing it. Note which way round the derivation runs: unlike broso, where the thing is the root and the verb is built on it, here the sport was borrowed as the root and the object comes off it. There is no rule that tells you which way a given root faces, and that asymmetry is the honest asterisk on the one-root-four-words promise. The -um wildcard and word-formation sets out why.

Skii is one of a small family of sport loans that came into Esperanto the same way and behave the same way: futbalo, tenisi, boksi, golfo. They are transparent, they need no explaining, and they are quietly one of the strongest arguments for the language's method: a planned vocabulary does not have to invent a word for skiing when the world has already agreed on one.