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Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?

Just starting out with lacrosse pictures on my new Canon EOS 60D and zoom lens EF 70-300mm f/4.0-5.6 IS USM. Why can't I set aperture at 4.0 and it stay there? Why does the aperture get smaller as I zoom in? I just discovered that this happens. I just thought I couldn't dial to 4.0 and couldn't figure out why wouldn't go any larger. Doing all in Manual mode. I hope this isn't as stupid as it sounds, but I think when I learn the answer I will have a lightbulb moment and really grasp this. Thank you for anyone's input.

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    pretty much an exact duplicate of photo.stackexchange.com/questions/14492/… and photo.stackexchange.com/questions/13918/… – ElendilTheTall Apr 16 '12 at 18:06
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    Welcome to the site, Michelle. It's not stupid at all, and hopefully the links ElendilTheTall provided will answer your question. If not, or if you have others, feel free to ask. This is a question and answer site, so apologizing for asking is never necessary. – Please Read Profile Apr 16 '12 at 18:12
  • You will need a premium lens such as the "L-series" 70-200mm to maintain a constant aperture. – Jakub Sisak GeoGraphics Apr 16 '12 at 18:47
  • @Jakub: sorry to nit-pick, but that's not quite right. There's at least one Canon zoom that is constant aperture throughout the zoom range, but is not an L: the 17-55 F/2.8. It is true that most of the "L" zooms are constant aperture, but not all of them are (for example, the 28-300 f/3.5-5.6 L and the 100-400 f/4.5-5.6 L). – djangodude Apr 16 '12 at 23:42
  • No worries @djangodude. You are right. Thanks for correcting that. I didn't mean to imply that he must buy an L lens. Personally, I would consider the great 17-55 f1.8 a premium lens although it is not an L. – Jakub Sisak GeoGraphics Apr 17 '12 at 0:01

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