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: An example of combining two substances is pouring one substance into the other substance
An example of combining two substances is?

Fact: Matter in the gas phase has variable shape
A solid has a fixed shape. When liquid is boiled and turns into gas, its shape is?

Fact: grey clouds appear before precipitation

In the intermediary before rain falls, observers may note?