Bubbles in the water can be produced either by aerating it mechanically (in some cases using jets that also move the water) using equipment installed permanently or temporarily in a bathtub, hot tub, or pool, or by producing gas in the water in a bathtub through the use of effervescent solids. The latter can come as small pellets known as bath fizzies or as a bolus known as a bath bomb, and they produce carbon dioxide by reaction of a bicarbonate or carbonate with an organic acid. Fizzing bath products came into use as effervescent bath salts early in the 20th century; the bath bomb became a popular form late in that century.