"Flexible" [folk form]

"it's a lucky thing you're flexible!
  'twould be a hazard
if you should grow inflexible"
  I told the buzzard

"it's fortunate you're flexible!
  'twould be a danger
if you should prove inflexible"
  I told the stranger

"it's wonderful you're flexible!
  'twould be arthritic
if you should turn inflexible"
  I told the critic