Learn about 'testable questions'

A testable question is one that can be answered through hands on experimentation by the student. The key to a testable question is to find something where you change only one thing (or a very small number)

Examples:

Untestable: What makes something sink or float?
Testable: How well do different materials sink or float in water?

Untestable: How do rockets work?
Testable: How does changing the shape of a rocket's fins change its flight?

Untestable: What makes a magnet attract things?
Testable: Does temperature have an effect on a magnet's strength?

Untestable: What happens when water expands as it freezes?
Testable: How much force is needed to keep water from expanding as it freezes?

Untestable: What is bread mold?
Testable: What conditions keep bread mold from growing on bread?