Indeed, overtightening is the most common cause of leaks in compression fittings. A good rule of thumb is to tighten the nut first by hand until it is too difficult to continue and then tighten the nut one half-turn more with the aid of a wrench; the actual amount varies with the size of the fitting, as a larger one requires less tightening. The fitting is then tested: if slight weeping is observed, the fitting is gradually tightened until the weeping stops. The integrity of the compression fitting is determined by the ferrule, which is easily prone to damage. Thus care should be taken to when handling and tightening the fitting, although if the ferrule is damaged it is easily replaced. There are two traditional types of compression fitting, standard (British type-A/non-manipulative) and "flare" fittings (British type-B/manipulative). Standard fittings require no modifications to the tubing. Flare fittings require modification of the tubing with a special tool. Standard fittings are typically used for water and compressed air connections, whereas flare fittings are used for gas and high pressure lines. A standard fitting can be installed using an ordinary wrench to tighten the surrounding nut. To remove it, a specialized puller is often used to slide the nut and ferrule off the tube. If the ferrule is difficult to remove it can be weakened with a cut, care being taken to not nick the pipe while cutting. A newer type is push-in compression fittings. These fit over the end of a tube using a grip ring to hold the sides of the tube and an O-ring to form a seal at the end. Compression fittings are popular because they do not require soldering, so they are comparatively quick and easy to use. They require no special tools or skills to operate. They work at higher pressures and with toxic gases. Compression fittings are especially useful in installations that may require occasional disassembly or partial removal for maintenance etc., since these joints can be broken and remade without affecting the integrity of the joint. They are also used in situations where a heat source, in particular a soldering torch, is prohibited, or where it is difficult to remove remains of water from inside the pipe which prevent the pipe heating up to allow soldering. Compression fittings are not as robust as soldered fittings. They are typically used in applications where the fitting will not be disturbed and not subjected to flexing or bending. A soldered joint is highly tolerant of flexing and bending (such as when pipes knock or shake from sudden pressure changes).