Necessity may be the mother of Invention, but Spite can make for an exceptional foster parent. -- AmberFox
Is there any phrase more simultaneously motivating and de-motivating than "you can't"? Whether or not someone takes it to heart, or how they take it to heart, is a matter of importance.
"You can't do that, you simply lack the mental capacity," is a good one that has inspired many to prove the naysayer wrong. Of course, such proof is almost inevitably dismissed as a fluke by said naysayer.
"You can't do that, you're too small," has made many find a way anyway. Sometimes, this has not worked out so well, but others... well. They learn how to stand on the shoulders of giants, one way or another.