Bertie's Missing Rattle
Bertie's Missing Rattle is a side-quest in Skyward Sword.
After acquiring the Clawshots, you can return to Skyloft and enter Bertie and Luv's house. Sleep in the bed until night and then speak with Bertie, the husband, who is awake with the baby. He explains that their child has been cranky and unable to sleep ever since the rattle went missing. If you stumble across it, bring it back, will ya?
On the far east side of Skyloft there are floating islands (one of which has the iconic waterfall). Once you've acquired the Clawshots, you can use them to latch onto the vines and work your way from island to island to get on top of the big one. If it's night, you will also find the individual Gratitude Crystal #14.
Circle around to the south side and look straight ahead. The windmill is ahead of you, across the water. Get a running jump off the ledge and tilt the wiimote pretty far forward (but not forward enough to make Link dive) and glide to the windmill, which has a nest on top of it. Use the Gust Bellows to clear the junk off and you'll find the Rattle!
Sleep At Last!
Return to Bertie and give him the Rattle. That is one happy father! At long last, he can get some rest, meaning you just earned yourself five Gratitude Crystals!