How many times have your teams tried an idea only to find out that it doesn't work? The tower won't stand up. The bridge collapses. The boat sinks. If this happens to your team, be sure to give them the opportunity to try again. And if they're feeling really low, remind them that they haven't failed; they've simply discovered one way that doesn't work.
Point them to this post over at the Idea Sandbox that discusses "Sticktoitivity" and details just how many attempts James Dyson made before he became a billionaire selling those yellow vacuums. Or how many different versions of WD-40 were tried before it was successful. And then, tell your team to keep at it.