TO BE AS CUNNING AS A FOX:

FOXS ARE VERY CLEVER MAMMALS. WHEN SOMEONE SAYS: YOU ARE AS CUNNING AS A FOX. HE IS REALLY SAING THAT YOU ARE A VERY CLEVER PERSON AND THAT YOU CAN AVOID THE PROBLEMS.
FOXS WANT TO OBTEIN ALL THE FOOD THEY CAN, SO THEY'LL THINK ALL THE POSSIBLE ACTIONS NOT TO BE CAPTURED.