6. The frequency of a wave increases when the speed remains the same. What happens at the distance between two consecutive crests?

The speed of a wave is given by the formula:

v = f \cdot \lambda

where:
v = speed of the wave,
f = frequency of the wave, and
\lambda = wavelength of the wave.

Since the speed of the wave remains the same and the frequency increases, according to the formula above, the wavelength must decrease.

Therefore, when the frequency of a wave increases while the speed remains the same, the distance between two consecutive crests (wavelength) decreases.

\textbf{Answer: The wavelength decreases.}