What you're about to read is so gross that I've decided to sandwich it between two cute posts. I'm not sure that's even enough, so I'm sorry in advance.
Here it is - this is what you'll get if you don't brush your teeth regularly: black hairy tongue!
According to the Mayo Clinic, hairy tongue (lingua villosa) is a temporary (thank God!) and harmless problem resulting from an overgrowth of bacteria and yeast in the mouth. These organisms accumulate on the tiny projections of the tongue (called papillae) and cause the discoloration, which can be black, brown, white, green, pink or other colors (like after using colored mouthwashes and breath mints)
The main cause of the condition is poor oral hygiene, so brush your teeth regularly. Please.