The cream you squirt from a can onto cakes and desserts is liquid cream plus a propellant, usually nitrous oxide. The nitrous oxide gas is lipophilic (fat-loving) so it dissolves in the liquid cream in the pressurised can. Shaking helps the gas to dissolve.

When the valve of the can is opened, the high pressure inside forces the cream and the nitrous oxide gas out. At the lower pressure outside, the nitrous oxide bubbles out - just like carbon dioxide when opening a coke bottle - and mixes with the cream to create the whipped texture. It has the same effect as whipping cream by hand, except that it is mixed with nitrous oxide instead of air.

Nitrous oxide is chosen because it prevents bacteria growing and it doesn't oxidise the cream while in the can dispenser.

Michael Shelly, Edinburgh, UK

The cream on your strawberries is not a liquid, solid or gas. It is a foam: an intimate and relatively stable structure of gas bubbles held in a liquid matrix, stabilised by surface tension.

The proportion of gas by volume can be anywhere from 50 per cent to over 99 per cent, making the foam much less dense than the original liquid. But despite its high gas content, as far as its viscosity and surface tension are concerned it behaves more like a liquid.

By turning a liquid into a foam you can distribute a small amount over a bigger area or volume - hence their use in foodstuffs, cosmetics and pharmaceuticals. They are also useful for putting fires out - water based foams will float on the surface of oils, depriving them of oxygen. Foams with a very high proportion of gas are breathable because they contain such a small amount of liquid. So they can be used for blanketing fires in large spaces such as warehouses or ships' holds, without suffocating any people there.

Roger Calvert, Ulverston, Cumbria, UK