Why would a decreased length mean a higher frequency of sound? This is in terms of the Water Bottle Lab: adding water (effectively decreasing length) to a bottle and finding its fundamental frequency at different lengths/amounts of water.
I understand what happens, but why exactly does less length mean higher frequency?
 A: The sound wave in the bottle reflects off the water surface (with the reflected wave changing sign - so there is a displacement node there) and again at the mouth of the bottle (without changing sign - so there is a displacement antinode there).
Now resonance occurs when the frequency with which you stimulate the oscillations in the bottle "fit" - that is, when the wavelength $\lambda$ (which is a function of frequency, since sound velocity is independent of frequency: $\lambda = \frac{c}{f}$)  is such that $(2n+1)\lambda/4=\ell$. 
As $\ell$ gets smaller, $\lambda$ gets smaller and the resonant frequency gets higher.
A: Sound waves, like all waves, follow the Universal Wave Equation: speed = frequency x wavelength.  The speed of the sound waves is determined mostly by characteristics of the medium through which they are passing - temperature of the air for example.  As the bottle fills with water the properties of the air don't change (much) so the speed of sound stays the same.  As the bottle fills the length and volume of the air in the bottle decrease so the wavelengths of the standing waves in the bottle decrease.  Since speed is constant as the wavelength decreases the frequency of the sound must increase.
