Learn Finnish: Käteni on pieni.

When you add a possessive suffix like -ni (my) to a word, you must attach it to the word's stem. Finnish words ending in -si, like käsi (hand) or vesi (water), usually change that -si to -te- before adding endings. So, käsi becomes käte-, and then you add -ni to get käteni.

Where is the word for "my"? Shouldn't it say minun käteni?

In standard written Finnish, the possessive suffix -ni at the end of käteni is entirely enough to mean "my". You can include the word minun for emphasis (minun käteni), but it is optional for the first and second person (my, your).

Could käteni also mean "my hands"?

Yes! When you add a possessive suffix like -ni, the regular plural -t at the end of a word disappears. This means käteni looks exactly the same whether it means "my hand" or "my hands". We know it means "my hand" in this specific sentence because the verb on (is) and the adjective pieni (small) are singular.