What causes waves to be momentarily in phase and out of phase?

Waves being momentarily in phase and out of phase is primarily influenced by variations in their frequencies. When two waves have different frequencies, their wave cycles do not consistently align, leading to moments where their peaks and troughs coincide (in phase) and others where they do not align (out of phase). This phenomenon is often observed in sound and light waves, causing interference patterns such as constructive and destructive interference.

In contrast, waves of equal frequencies would maintain a consistent phase relationship, causing them to stay either in phase or out of phase at the same intervals. Differences in amplitude or speed do not inherently affect the phasing of waves; instead, they relate more to the energy or propagation characteristics of the waves rather than their synchronization. Hence, it is the variation in frequencies that facilitates the shifting phase relationship between the waves.