You can use slinkies to demonstrate all things waves! Start with the basics: wavelength, amplitude and frequency. Once you’ve got those down, then you can play around with some things that are a bit more complicated like wave type, standing waves, and superposition.
Longitudinal waves, like sound waves, expand and compress as they travel through a medium like air or water. Transverse waves are like ocean waves. They have characteristics that we more commonly associate to a wave: wavelength, the distance from peak to peak or trough to trough, amplitude, the height of the wave, and frequency, the number of waves per given time.
As you increase the frequency of waves, you can also increase the number of nodes and antinodes in the standing wave. Nodes are the part of a standing wave that stay in place. Antinodes are the part of a standing wave that move with maximum amplitude, like the peaks and troughs. There is always one more node than antinode.
You can also cause superposition of waves. Superposition is when you combine multiple waves together to get either constructive or destructive interference. To get constructive interference, the waves must be in phase. When call that coherent. Coherent light can often be found in lasers. The light waves travel at the same frequency and are in phase, which results in a single point of light. To get destructive interference, the waves would need to be sent out of phase, or be incoherent. Light from a white flashlight is a great example of incoherence, because the light has many different frequencies, resulting in a cone shape of light.
In the case of the slinky. If you send one wave down to try to knock the water bottle over, on their own they do not carry enough energy. But, if both ends of the slinky are sent in phase and work together, then we can achieve constructive interference to knock the water bottle down!
What other waves can you think of, and is there a good way to demonstrate them?