Lateral Thinking #16- Why Wash A Clean Face -

Two boys were having fun in the school playground at lunchtime. The school bell rang, signalling that lunchime was over. As they turned to head back to class, a gust of wind scooped up some dirt off the ground and blew it right in their face. When the wind stopped, one boy's face was covered with dirt, while the other boy's face was perfectly clean. But it was the boy with the clean face who rushed to the washroom to wash his face, while the boy with the dirty face went straight to class.

Assuming both children had equally good hygiene habits, can you explain this strange behavior?