Man.  In the morning of his life, before he has learned to walk, he uses four legs.  Once he has grown, he walks on two legs, upright.  In the evening of his years, he must rely on a cane, and thus has three legs. 
  
Sphinx'd 
  
Here's one: you throw away the outside, and you cook the inside.  Then you eat the outside, and throw away the inside.  WHAT IS IT?