Breathalyser. A 40 year old man blowing into the mouthpiece of a disposable self-testing breathalyser. Inside the mouthpiece are crystals that turn yellow when breath containing excess alcohol passes over them. A positive test suggests that the person has too much alcohol in their blood for them to be able to drive. It is compulsory to carry at least two such disposable testing kits in any road vehicle across much of Europe.