Of course, your assisant may or may not get tired of shaking after thirty seconds and declare that he is SO tired. And then he may leave you alone with your container of cream while he goes to torture the guy who is in your house trying to install your new satellite TV because Time Warner Cable has made you mad for the LAST time.
Don't let any of that bother you. Just keep shaking. Just keep shaking. Eventually, you'll make whip cream!
Just when you think your arms are going to fall off, just when you think you're going to quit, don't. That's when you'll notice things sound different in the container you are shaking. That's when the magic happens. You'll see butter forming and a liquid separating from it. (It took me about thirty minutes.)