If a "no tear shampoo" doesn't contain anesthetics (as urban legend suggests), then why doesn't it sting sensitive orbs? The answer lies in a few subtle changes in chemical formulas.
Adult and baby shampoos contain surfactants (short for "surface active agents"). One end of the surfactant molecule is attracted to water. The other is repelled by water but attracted to oily substances. Surfactants work by reducing the surface tension of a liquid, allowing the shampoo to spread and penetrate better, and remove the thin layer of oil known as sebum from the hair and scalp [source: Schwarcz]. Baby shampoos use detergents with long chain surfactants, such as sodium trideceth sulfate or nonionic polymers that are less harsh than normal detergents, and they use only small amounts of these cleansers in their shampoos.
Tear-free formulas also leave out surfactants such sodium lauryl sulfate, which can be irritating to the eyes and scalp. This does create a trade-off, though. Sodium lauryl sulfates -- formed in part from coconut fat or palm kernel oil -- are the chemical agents in shampoos that get hair really clean (and give a nice lather) [source: Schwarcz]. Although tear-free shampoos still clean hair, they don't remove oil as thoroughly. But since most babies don't do more than look cute and occasionally smear food into their hair, this usually works out just fine.
If you want a baby shampoo almost chemical-free, you might try making your own. It's easier than you think. All you need is castile soap -- a type of soap made exclusively from plant oils -- diluted 1-to-3 with water [source: Care2].
Keep this in mind, though: The purest shampoo out there doesn't necessarily mean you won't see any little tears at bathtime.
Your baby's bathwater -- without the use of baby soap or shampoo -- could still cause tears. It all depends on the pH level of the water, which is a measurement of the free hydrogen and hydroxyl ions in the water. A pH level measures water on a scale of 0 to 14, with 7 being neutral. If the number drops below 7, the water is increasingly more acidic, which means it has a greater amount of free hydrogen ions. Above 7, and the water is increasingly less acidic (or more basic), which means it has a greater number of free hydroxyl ions [source: USGS]. The pH range for human eyes is 6.5 to 7.6, with 7 being optimal. Any variation from neutral may cause your baby's eyes to tear, and it has little to do with the tear-free shampoo [sources: Kiechle, WHO].