|I'm sorry, I thought you said "witch's hat"|
"That" is a restrictive pronoun.
"I like shoes that are expensive."
There are all kinds of shoes, expensive and inexpensive, but we're only talking about the shoes I like and that is restricted to expensive ones. "That" qualifies my taste in shoes as liking only one kind out of many varieties.
"I like diamonds which are expensive"
Let's say we're talking about the diamonds DeBeers constantly harasses us into buying with their propaganda ads for engagement rings needing to cost three months salary. In this case, there are all kinds of these diamonds, all of which are expensive.
Here's a sentence using "which" and "that" so you can see why one is right and one is wrong.
"I like shoes that are expensive". I like shoes, but only restricted to expensive shoes.
"I like shoes which are expensive". I'm saying here that I like shoes and also all shoes are expensive. Wrong, I once bought a pair of Jones New York pumps for $10 bucks. That's an expensive shoe for an inexpensive price.