I think you just answered your own question. Maybe you can toss in a little 'gas' too if you want, just for the sake of argument.
so if the tail is made of rock and ice falling off the comet, then is the comet continually falling apart? Why doesn't it erode away to nothing then?
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They eventually do if the Sun or Jupiter's gravity doesn't get them first. Nothing lasts forever in this universe, not even comets.