Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: I just read about using cardboard in layers for building, but the surfaces should be waxed to keep out water.  This premise is used by some _____________ to keep water in:
leaves
ponds, pools, or wetlands.