sentences: helium is less dense than air & a balloon is a kind of container & a balloon is filled with helium gas ->  a balloon filled with helium is an example of a container contains a substance less dense than air  a balloon filled with helium is an example of a container contains a substance less dense than air & if a container contains a substance with less density than air then that container may move upward -> the balloon filled with helium may move upward if released