Build up of residue inside your pieces will cause mold and other fungus'. The ash and wax has a lot of Nitrogen and Carbon that gets released into the water (for water pieces, and non-water pieces due to typical humidity levels) which promotes life/growth. You should rinse them out every couple uses, and if you see residue building up along the inside walls/once a week (even for light smokers) follow the cleaning procedure below:

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• Rinse thoroughly with hot water.
  • I let mine sit in the sink with running hot water into the top so it flows out the bottom in a stream for a few minutes.
• Put some rubbing alcohol in there. (I try to use 90%, but 70% works too.)
• Cover both holes and gently shake for a minute. (This step is optional, unless it's really gross in your piece)
  • You can usually create a decent seal and still have a good grip using your palms.
  • Rinse again.
• Pour in coarse/table salt and more rubbing alcohol.
• Cover holes, gently shake the piece.
  • The salt 'scrapes' the gunk off the inner walls.
• Let it sit for a little bit soaking in the alcohol.
  • The more gunk, the longer it sits.
• Rinse with hot water.
• Add some Dawn (or other grease cleaning dish soaps) and a little hot water, then shake.
  • This is to get any residual alcohol out, and break up the last little bit.
• Rinse soap out!

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If you need to clean a small pipe, and covering the holes isn't feasible, put it in a Ziploc bag and shake that up.

Repeat this process until clean. For me, the whole process above takes about 10 minutes and works much better than those expensive 'cleaning kits' you get at a headshop.

Don't forget pipe cleaners! They're cheap and well, designed for cleaning pipes.

Be safe, stay clean, marijuana is safe, but not if you've got colonies of bacteria or mold in your pipes.

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EDIT: A user suggested to me that rubbing alcohol can be detrimental to acrylic pieces. This post only applies to glass! If people know the best way to clean acrylic, please share with the rest of us. I don't have much experience with acrylics so I'm not going to give advice on cleaning those.

You're adding absolutely nothing to the conversation and it just appears like you're trying to farm karma... Yet it's normally the first comment on this sub

Inb4, take my upvote