a balloon is a kind of container & a balloon is highly flexible ->  a balloon is a kind of highly flexible container  a balloon is a kind of highly flexible container & a weather balloon is a kind of balloon ->  a weather balloon is a kind of flexible container  a weather balloon is a kind of flexible container & as air pressure surrounding a flexible container decreases , that flexible container will expand ->  if the air pressure surrounding a weather balloon decreases, the weather balloon will expand  if the air pressure surrounding a weather balloon decreases, the weather balloon will expand & as altitude increases , air pressure will decrease -> the weather balloon expands because the air pressure decreases at higher altitude