The bear who swallowed a fly

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A bear is sleeping peacefully in the woods when he accidentally swallows a fly. He can't stand it buzzing around in his belly. On the advice of a squirrel, he swallows a frog... who croaks inside him; then a grass snake, whose wriggling tickles him; then a heron, whose beak prickles him. What lengths will he go to to stop this torture?