Before you wash those empty bottles of barbeque sauce or mustard or Worcestershire sauce to get them ready for recycling, be sure they really are empty.
Pour about 1 tsp of vinegar into the bottle, swirl it around and then empty it into an almost empty bottle of Ketchup. (I save those for just this purpose.)
This past weekend we used up the last of a bottle of barbeque sauce. And I remembered that I had maybe one squirt of mustard left in the bottle in the fridge. So out came the vinegar and I got to rinsing.
You may have to rinse the bottle with vinegar a couple of times to get out that last little bit of goodness. Then once you’re done, put the lid back on the Ketchup bottle and give it a good shake, label it and put it back in the fridge. Add more bottom-of-the-bottle goodness as you use things up.
Then when you’ve got about of cup or so, use that as a barbeque sauce. Or what I most often do is put some frozen chicken pieces into the slow cooker, pour the homemade sauce over it and let it cook all day on low. The chicken comes out fall off the bone tender and juicy and so flavourful.