Rolling in the grass

The weather has been nice lately.  Often when I pet Daisy she will lay on her side so I can better scratch under her chin.  And then she will roll over so I can scratch her from the other side.  When she does this outside she gets a little dirty from last year's dead grass.   When I try to pet/brush the grass off her she will then lay back down before I finish brushing.  So I gave up and let her clean herself later.