A disadvantage of many kinds of ink is that they may smudge when wet, spoiling the picture or writing. If water-based ink is used, the writing situation needs to be stable, with the writer seated at a table. Ink in a ballpoint pen (biro) is a kind of gel. It is held in a thin long cylinder (tube) inside the pen. The ink does not fall out of the cylinder as it sticks to the sides of the tube. Therefore ballpoint pens can be used in a wider range of circumstances compared to water-based inks.