A 2nd-grade teacher wants to introduce the principles of invisible energy. Which material will she need?

The choice of using iron filings and a magnet is a foundational approach to introducing the principles of invisible energy, specifically magnetism. When iron filings are sprinkled around a magnet, they align themselves along the magnetic field lines, visually demonstrating an invisible force. This hands-on experience helps students observe how magnets exert force without direct contact, thereby illustrating the concept of invisible energy in a tangible way.

The other materials mentioned do not effectively convey the concept of invisible energy. Plastic and a ruler, for example, might be useful in exploring physical measurement but do not demonstrate energy or force principles. Water and a container can be beneficial for various experiments but do not specifically relate to invisible energy. Lastly, paper and a pencil are essential for note-taking and drawing but lack the capacity to convey the concept of energy. Thus, using iron filings and a magnet provides a clear, engaging, and educational demonstration of invisible energy principles suitable for 2nd-grade students.