Learn German: Meine Ohren sind kalt.

Ohren is plural (the singular is das Ohr). Because it is plural, the possessive word mein needs the plural ending -e, making it meine.

Wait, I thought you say Mir ist kalt for 'I am cold'. Can I just use sind kalt for body parts?

Yes! When talking about your whole body feeling cold, you use the dative structure Mir ist kalt (literally 'To me it is cold'). But when you talk about specific body parts being physically cold, you can just use the normal subject and verb structure, like Meine Ohren sind kalt.

Is the singular of Ohren a feminine noun, since it ends in '-en' in the plural?

No, the singular is actually neuter: das Ohr. It is an exception to the normal plural rules. Most neuter nouns add -er or -e in the plural, but das Ohr adds -en to become die Ohren.

Why doesn't kalt get an adjective ending here?

In German, adjectives only take an ending when they come directly before a noun (like meine kalten Ohren). When the adjective comes after a verb like sind (are), it stays in its basic dictionary form without any endings.