Will It be Possible to Sin in Heaven? Why Not?

You all know the “right” answer to the question—of course it will not be possible to sin in Heaven—but I have found that many believers are not sure why it won’t be possible sin in Heaven. After all, they say, there was not any sin in Heaven before Lucifer rebelled, and there wasn’t any sin on Earth before Adam and Eve ate the fruit. We know that no sin will enter Heaven (Revelation 21:27), but what will keep me, or others, from sinning once we are in Heaven?