Mice are nocturnal animals that have made amazing adaptations to life in the dark. They have very sensitive whiskers (vibrissae) and an excellent sense of smell. To humans carbon dioxide is an odorless gas, but mice have special CO₂ receptor molecules in their noses that can detect an increase in carbon dioxide. Perhaps they have developed this ability to detect the respiration of an approaching predator. Minmin Luo at the National Institute of Biological Sciences in Beijing trained mice to lick water whenever they detected an increase in CO₂ levels. Don’t ask me how he trained them or why he chose to train them to lick water. The atmosphere typically contains 0.03% carbon dioxide, mice can detect CO₂ concentrations of more than 0.06% CO₂ in the atmosphere. In complementary work Peter Mombaerts at Rockefeller University has created a strain of mice in which the guanylyl cyclase-D expressing neurons are labeled with GFP. His research has shown that these olfactory neurons are activated by exposure to carbon dioxide.