Obviously you don't want to say, "I'm happy to waste away forever in a job that rarely tests me." So if you aren't sure where you'd like to end up down the road, say: "I'm excited to learn as much as possible about your organization while I do my job every day. I'm confident that as time passes and you see how well I contribute, the company will offer me other opportunities." It'll help to show you've already proven you can grow in earlier jobs as required.