I have had my Mellowdog for a couple of years and love it. But the mouthpiece has become so loose that I can’t keep it in tune. It will actually fall off in my hand. Does anyone have a fix for this? I would really appreciate any advice.
Jodie in NM 
Every whistler/fluter should own a few rolls of teflon plumber’s tape. Less than a buck at your faovorite hardware store.
One issue with teflon tape is that because it’s slippery, it’s easy to put too much on and split the outer part. Just something to be aware of if you’ve never used it.
If I had a slightly loose whistle head, I think I’d rub some beeswax on the end of the tube.
I’d go with the beeswax as well … good stuff, just polishes off if you don’t like it. Before you start, unless you’re in the habit of moving the whistle-head frequently and may have worn it loose, look for a crack in the plastic. It’d be unusual for a whistle-head to become that loose without good reason. If it is cracked, packing the joint with tape, wax etc. may make it worse, not better.
Hi Jodie
Kypfer has a point…Over time, plastic whistle heads become brittle. This is especially true if exposed to heat and sunlight (UV rays). If you do find a crack in the headjoint, you can fix it easy. Wrap the head below the voicing window with nylon thread until it fits tight around the whistle body again. Coat the thread wrapping with clear nail polish and you have an attractive colored wrapping of your choice and a permanent fix.
I have a much older Mellowdog that also developed a tiny crack. I applied a couple of drops of Superglue with the mouthpiece off the tube so that it was not stressed. Has been going strong for a couple of years or more.