In the days when people used bars of soap, we always stuck the last sliver of the old bar onto the new one.
In fact, Pears soap (anyone remember that, it was transparent) actually ENCOURAGED it, explaining that was what the hollow in the soap was for.
I never dilute things like shower gel to get the dregs out, instead I put a funnel into the new bottle then turn the old one upside down into the funnel. Sometimes it takes hours to drain out, but eventually there is no waste at all.
And I cut open toothpaste tubes and can get a few days use out of what is stuck inside.
Also, when I boil a kettle, I put any surplus water into a vacuum flask, which keeps hot ebough for most uses, or I can pour it back into the ketle to reheat, it stays hot for about 10 hours or more and saves a lot of electricity. I don't heat up more than necessary, but you have to fill it to the minimum level, which is more than a mug full for making tea etc.
Hmmm. Used to?
I was sticking the last sliver of my old bar onto my new one earlier after I'd been cutting open the nearly empty toothpaste tube (no joke).
I do dilute the last dregs of shower/hand gel/shampoo as I like the way it gets the rest of my gel super bubbly (I use a Japanese wash cloth that does that anyway but super bubbly is always nicer).
It's more a waste-not want-not thing with me. Old habits die hard.
I boil a minimum of two cups when I boil the kettle as I'm a tea addict, have a massive mug and always make it in the pot so only one teabag needed.
I'd suggest bigger mugs, Strato. Save the hassle, enjoy more tea. Win win