Valuable

 There were these people who thought their thoughts were very valuable and so, wherever it was they were going, or thought they were going, they tried to take their thoughts with them, kept them in sacks or whatever, maybe tied them around their waists. Unfortunately though, when they came to an obstacle, a body of water, a bit of a river that they had to cross, instead of keeping them afloat and helping them across, their thoughts dragged them down.