LPG is a colourless liquid that evaporates into a gas. It has no smell, but does have an odour added to help detect leaks. It can burn or explode when it mixes with air in the right proportions and finds an ignition source. It is heavier than air. It can flow for long distances along the ground, and can collect in drains, gullies and cellars. LPG is supplied in pressurised cylinders to keep it liquefied. The cylinders are strong and not easily damaged, although the valve at the top can be vulnerable to impact. Leaks can occur from valves and pipe connections if the connections are not kept in good condition or if sufficient care is not taken when changing cylinders.