i don't recycle leftovers, heat oxidizes and breaks down organic compounds, what remains will be tainted, whenever i have done this the taste was unpleasant and the effect uncomfortable.
Always put more than you need in a pipe so you get the full effect, think of what remains as a necessary sacrifice to help you get to where you need to go, it's as much part of the process as the medicine that is vaporized.
Consider the number of molecules in your pipe that actually reach the receptors in your brain, a very small percentage, most will be lost to the atmosphere or just not used, but they all work together to deliver the overall experience. I always return the remains to a flower pot and let nature recycle it that way.
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I am not a number, I am a free man.