Well, exactly, Matthew. This is just a ridiculously puerile attitude! If you have an idea of something else to do, or a different way of doing it, the supervisor should discuss it with you, and explain to you why it's not such a good idea. If you're still convinced it'll work, they should let you do it - if it doesn't work out, they should be there to get you back on track.
There's obviously limits, and they should tell you (nicely!) if you're just being an idiot, but you're still learning, you may be passionate and thoughtful about your subject, and may just be over-imaginatvie. Nothing wrong with that at this stage.