Given a fact, create a question that can be answered using the fact. Construct the question such that it is unambiguous, has a unique answer and the answer can be given using the fact.

Fact: processes sometimes produce waste products
What product eventually will become human waste?

Fact: some birds live in forests
Some birds live in?

Fact: weathering usually occurs over a period of many years

A stone is weathered into sand in about?