A bucket in the cave

An empty bucket lies on the ledge in the cave. What's it for, and where's the water?
Nudge 1Where to look
The cave has pools at the bottom.
What you needOther problems to solve first
Solve these first (each opens its own page, no answers on the way):
Show the solutionThe whole answer, with a video
Pick up the empty bucket (it lies beside the trampet: B takes the trampet first, so take both and put the trampet back on the ledge afterwards). Walk down into one of the cave's pools with it: it becomes a bucket full of water. It puts out a fire in Zaks' castle (A fire on the castle's lower level).