In that particular instance, I'd say, "It's easy! Just learn the basic stitches, then practice for X years."
In most instances, I just don my best innocent look and say, "Why can't you?" Which may be followed by, "How long have you tried?" I do beaded micro-macrame and also sculpt OOAK art dolls, and people are always telling me this...or the, "I'd love to do that stuff, but I don't have the time." I think it's complete horse...cookies...and usually tell them so in only a slightly nicer way. You can find time for whatever is important to you -- this is what I do in lieu of watching TV (or while I'm watching a movie) and I don't waste my time gaming online, I don't spend countless hours with friends, and I have an intense aversion to spending time on the phone unless there is a very definite goal for the phone call.