If your LO develops a new interest in something, what do you do to support it?

My LO go through phases, but they tend to stay in certain phases for a while, so I don't feel bad going all out and really getting into their new interests with them. It gives us something to do and and if it's collecting something, that's fun as well. We are currently in a Little Critter phase, and trying to collect the books, new or used! Used makes it more fun, because we can keep our eye out at thrift shops.