helium is less dense than air
helium is lighter_air. helium_gas is heavier_than_hydrogen_gas. helium is not lighter_than_air. helium_balloons is not lighter_than_air. helium_gas is not heavier_than_hydrogen_gas. helium is not seven_times_lighter_than_air. helium is lighter_than_air. helium is seven_times_lighter_than_air. helium_balloons is lighter_than_air. helium is not lighter_air.