Learn Russian: У меня два колена.

The word for knee is колено, which is a neuter noun. The number two has different forms depending on gender: два is used for masculine and neuter nouns, while две is only used for feminine nouns.

Why is the word for knee written as колена here?

After the numbers 2, 3, and 4, Russian requires the noun to be in the genitive singular case. The dictionary form of the word is колено. To make it genitive singular, the neuter -о ending changes to -а, giving us колена.

I thought the plural for knee was колени. Why don't we use that here?

You are right that the plural of knee is колени! However, because we are using the number 2 (два), grammar rules force us to use the genitive singular form (колена) instead of a plural form. You would use колени if you were just saying "knees" without a specific small number attached.

Where is the verb in this sentence?

In Russian, the normal way to say "I have" is using the structure у меня (literally "by me"). In the present tense, the verb "is" or "are" is completely left out. The sentence literally translates as "By me [are] two knees."