Is this a trick question? <img src="/images/graemlins/tongue.gif" alt="" />

CK has the best answer: find some clay. Dig a gallon-size hole, line with leaves, line with clay, build fire inside clay to fire it. Cool. Lift this bowl from the soil/leaves, carry to water source and fill. Meanwhile, you've built another fire & heated some rocks that you didn't collect from along the river/lake. Return filled bowl to camp, use forked green branches to lift hot rocks into bowl of water until boiling.

If you're incredibly lucky (dumb, but lucky), you might find a water-eroded rock bank along the shore that has a ready-made bowl formed in it above the water line. Splash water into it, build a fire as near as you can, heat rocks and drop into the water until boiling.

Sue