Learn Polish: Skórka jest cudowna, ale bolą mnie ręce. - The crust is wonderful, but my hands hurt.

In Polish, the verb boleć (to ache / to hurt) takes the accusative case for the person experiencing the pain. So you say boli mnie głowa (my head hurts me), using the accusative mnie, rather than the dative mi which is used for things like giving or seeming.

Why is the verb bolą in the plural form here?

The subject of the second clause is actually ręce (hands), which is plural. In Polish, body parts that hurt are the grammatical subjects doing the action of hurting. Literally, the sentence translates to "hands are hurting me," so the verb must match the plural subject.

Is skórka only used for the crust of baked goods?

No, skórka is literally "little skin" (the diminutive of skóra). While it perfectly translates to "crust" for bread or pizza, it is also the standard word for the peel of a fruit, like an apple or a lemon, or the skin of a vegetable.

I remember the singular is ręka. Why isn't the plural ręki?

The word ręka is a historical exception that kept an older form for its plural, becoming ręce in the nominative. This is a common pattern for paired body parts in Polish, just like oko becomes oczy and ucho becomes uszy.