fluid. When unscrewed, the brush is covered in liquid Tipp-Ex which is then painted over the mistake. However, it is still possible to see that the product has been used, and also the original content if the paper is held up to a light. If the contents of a Tipp-Ex bottle have been exposed to too much air (for example, if the cap has been unscrewed too long or if the bottle is half-full and there is air filling the space), the fluid becomes thicker, and sometimes even solidifies. To help prevent this, Tipp-Ex released a product called "Tipp-Ex thinner", a few drops of which are mixed with the contents of a bottle of partially dried Tipp-Ex to thin it. This extends the lifetime of a bottle. One must write with a ball point pen on top of Tipp-Ex, as a liquid ink pen will smudge. Gel ink will require a few seconds to fully dry, but can be used if no ball point pen is available. Toxicology The organic solvent 1,1,1-trichloroethane was originally used as a thinner in the product. This solvent is readily absorbed by the lungs via inhalation. It passes readily through the blood–brain barrier and may be lethal following acute exposure. 1,1,1-trichloroethane is also an 'ozone depleter', being one of the substances responsible for causing damage to the Earth's ozone layer. For both reasons, it has not been added to Tipp-Ex since 2000, and it has been replaced by aliphatic hydrocarbons. See also Liquid Paper Wite-Out References Correction instruments Stationery