increasing K value raises question

[beardyphish]

As the weather is slowly getting warmer around here (yup, the increasing K value of the subject is the temperature in Kelvin ) I was reminded of a simple riddle. So;

What is

- black when you buy it
- red when you use it
- gray when you throw it away ?




well if our leader adds a subject "war arguements" (his spelling error, not mine ) then I can add a non-prime topic too I feel

[Frodo42]

How about tobacco?

[eatmadustch]

how about coal?

[Stricker]

charcoal you know for a barbque