One of the many oddities of liquid helium is that it often seems to behave as though it were a gas. It does not, of course, expand to occupy all the available space in its container - it could not be described as a liquid at all, if this were the case - but in terms, for example, of its very low density, its refractive index close to unity, its feeble interatomic forces, its tiny surface tension and its extremely small viscosity (less than that of air at room temperature, even for normal, non-superfluid helium-4), its properties often seem more gas-like than liquid-like.