Answer:
Forward I am heavy backward I am not. What am I?
The dirtier I get, the whiter I am. What am I?
The more holes you cover the lower I go.
Draw fire or fill me; I'm still empty.
Though desert men once called me God today men call me mad. For I wag my tail when I am angry. And growl when I am glad.
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.