OK, somebody straighten me out, I am totally done in by thinkin’. If a whistle blows sharp is it because the tube is too short or too long and why? I know I have the IQ of a retarded zucchini when it comes to this stuff, but in one or two other areas that are totally not related, I can be brilliant when I’ve had enough sleep!
You almost have 1,000, Anna!
A whistle is all-around sharp because the tube is too short. The shorter the whistle, the higher it gets.
Pulling out the mouthpiece some (if possible) brings it into tune.
Now I do! 
I had that whole thing bass akwards!
Anna,
the Longer the whistle the Lower it sounds. Think of the different sizes of whistles with the teeny ones being squeaky and the long low D’s sounding low. If your instrument is sharp - that is it sounds too high, lengthen it to lower it, usuallyl by pulling out the mouthpiece a bit. If it is only a bit out of tune or out of tune only on certain notes, you can sometimes fix the problem by blowing harder or quieter.
Anna, the pitch at which a whistle plays is determined by the length of the tube, as you figured out. This is because the air in the tube vibrates at a wavelength (pitch) related to the length of the tube. If the tube is longer, like for a trombone, the lowest note you can play has a very long wavelength and a very low pitch. If the tube is very short, like a Gen G, the pitch is very high indeed. Have you ever noticed that a low D whistle is twice as long as a high D? It turns out that if you double or half the length, doubling or halfing the wavelength, you produce the next lower or higher octave. That is also why the top tone hole on your whistle is half way along the tube. (OK you physicists out there, blast me with Gen G. This is meant to be a simple explanation for the non-scientists out there.)
NancyF
Congratulations, Anna!
Jerry, Jerry, Jerry! Is that because I finally hit a grand in posts or because I finally admitted I was a total idiot when it comes to whistle physics? I cop to both dyslexia AND dyscalculia, so my brain literally can’t function along those logical lines… and I only remember I have it when it comes to whistle physics. Now why do I bother with even thinking about it? It’s PAINFUL!
Zucchinis are very smart vegetables. Even retarded ones are much smarter than your average turnip (when they’ve had enough sleep).
At least they don’t wiggle when you eat them! All turrnips need to be turned into COMPOST for smart veggies!