It's show-and-tell with the new app sensing if you're over drunk to drive by looking into your eyes.
The Breathaleyes app detects rapid eye movement caused by drinking using an iPhone camera and tells you how drunk you.
Its maker claims the test is accurate to within 0.02 per cent of a breathalyser test: the 'HGN' test, tracking horizontal eye movements caused by alcohol, is one that's admissible in court in several American states.
"We think it's a great tool to help people make informed decisions when it comes to alcohol safety," the Daily Mail quoted Rob Andrews, one of the co-founders of the app, as saying.
The technical term for rapid eye movement after drinking is 'Horizontal Gaze Nystagmus' (HGN).
The makers of the app describe this as 'a condition of the eye indicated by the involuntary rhythmic oscillating motion of the pupil left and right across the horizontal field of view'.
They add that Nystagmus can be the consequence of any disease of the brain, sedative effect, stroke, or elevated levels of blood alcohol content'.
The app uses a mobile phone's camera function to take a picture of someone looking as far as they can to one side, it then measures any tiny movement the pupil is making.
The kind of breathalyser the police use on motorists only work 20 minutes after the consumption of alcohol, but this app works instantly.
The developers emphasise that the app is strictly for entertainment purposes only, with HGN also being caused by certain medicines and high altitude.
But road safety experts warn this hi-tech version of blow-in-the-bag testers might let you down.
"If you test when you leave a pub car park, your blood alcohol levels will rise for about 40 minutes - so you could pass a BreathalEyes test, then fail the real thing half an hour later," said Andrew Howard, the Automobile Association's Head of Road Safety.
"Technologically speaking, this seems more likely to be warn you that you couldn't drive when you could - it doesn't just detect if you're drunk, it also detects if you're impaired by anything, even a cold, or a twitch, or just that there are too many bright lights when you do the test," he added.