A marker pen, marking pen, felt-tip pen, flow or marker, is a pen which has its own ink-source, and usually a tip made of a porous, pressed fibers such as felt. A typical permanent marker consists of a container (glass, aluminum or plastic) and a core of an absorbent material such as felt. This filling serves as a carrier for the water-proof ink. The upper part of the marker contains the nib that was made in earlier time of a hard felt material, and a cap to prevent the marker from drying out. Until the early 1990s the most common solvents that were used for the ink were toluene and xylene. These two substances are both harmful and characterized by a very strong smell. Today, the ink is usually made on the basis of alcohols (e.g. 1-propanol, 1-butanol, diacetone alcohol and cresols).