Electromagnetism

Principle: A current carrying coil placed between the magnets starts rotating
Applications:
All Motors fundamentally work on the same principle

Items Required to make:
Copper wire (6-9 feet)
PVC pipe ½ inch to wind the wire in the form of coil
Magnet (preferably a bar magnet)
Power source Battery
Jumbo jump clips
Plastic glass
Connecting wires
Fun Facts to know: Bullet trains work on Magnetic levitation!
Electromagnets are used in Magnetic Resonance Imaging for Medical Diagnostics

Retpocileh

Principle: The rotating blades of a helicopter push the air downwards creating a lift. When the Retpocileh is released it starts rotating. The angles of the blades
Applications:
From the most common Household appliances (like exhaust fans, coolers, etc) to the highly sophisticated devices (like turbo propellers, gas turbines, windmills, etc), fans are the primary components.
Items Required to make:
Paper strips of different types
Straws
Different material

Possible Experimentation:
Number of blades,
Different sizes of blades (Length and Breadth)
Length of the Stem