Personal lubricants (colloquially termed lube) are specialized lubricants used during human sexual acts such as intercourse and masturbation to reduce friction to or between the penis and vagina, anus, other body parts, or applied to sex toys to reduce friction or to ease penetration. Surgical or medical lubricants or gels, which are similar but not usually referred to or labelled as "personal" lubricants, may be used for medical purposes such as speculum insertion or introduction of a catheter. Water-based personal lubricants are water-soluble and are the most widely used personal lubricants. The earliest water-based lubricants were cellulose ether or glycerin solutions. Products available today may have various agents added for even dispersal, moisture retention, and resistance to contamination. The viscosity of these products can be altered by adjusting their water content and concentration of cellulose or other gel-forming hydrophilic ingredient. Because water-based personal lubricants absorb into the skin and evaporate, most water-based lubricants have a tendency to dry out during use, but reapplication of the lubricant or application of water or saliva is usually sufficient to reactivate them. When the lubricant eventually dries out, it may leave behind a residue derived from the other ingredients in the formulation.