Sentence:  Now she kept herself protectively ready to laugh again and sure enough he pointed at her with his index finger and said `` Toot '' ! once more and roared off into the <WSD>fog</WSD> , his foot evidently surprising him with the suddenness with which it pressed the accelerator , just as his hand did when he worked . 
Meaning of the word marked with <WSD> tags:  droplets of water vapor suspended in the air near the ground