Sylvester And The Magic Pebble by William Steig

Sylvester loves to collect rocks.  She finds a particularly beautiful one that turns out to be magic.  When she sees a lion she wishes herself into a rock to protect herself.  But then she cannot turn herself back into herself because she drops the pebble!  One day her mother and father have a picnic lunch right on her.  Her father picks up the magic pebble and places it right on Sylvester the big rock!   And then Sylvester thinks, “I wish I were my self again, I wish I were my real self again!”  And in an instant she was.  You can imagine the hugs, the tears, the embraces that then took place.  The family took the magic pebble and put it in an iron safe.  Maybe someday they would use it.  But for now they had all they wanted.