How long could you survive without your phone?
Psychologists have found that the answer is probably only a few minutes - at least among people aged 18 to 26.
In a study, people whose phones were taken away from them were more likely to show "stress behaviour" than those who had their phones on them.
Participants who were given another mobile showed less signs of stress too, even though it wasn't their own phone.
The comfort from the mobile phone is a substitute for real human interaction, say the researchers.
They even compare it to how a baby can be comforted with a blanket when they are away from their parents.
The study was conducted by scientists from Eotvos Lorand University in Hungary and is published in the journal Computers in Human Behaviour.
Veronika Konok, one of the authors, says: "Objects can be the subject of attachment, like photos of important people, or toys.
"The mobile phone is special because it's not only an important object, but also represents our other social connections."
The study was done with a group of 18 to 26-year-olds, who were filmed and whose heart rates were monitored.
Half of the group had their phones taken away and put in a cupboard.
Each of the 87 participants would sit in a room on their own and asked to do sums and puzzles on a laptop.
In the three-and-a-half-minute break between activities, those without their phones were more likely to hover around the cupboard where their phone was, and within that time showed signs of stress such as a change in heart rate variability.
Those without phones were also likely to fidget, touch their faces or scratch - all signs of stress, according to the psychologists who ran the experiment.
The subjects were also tested for their reactions to emotive words, and were found to respond more to words to do with separation, for example "break up" and "loss".
Veronika adds that she thinks that young people feel a stronger connection with their phones: "Children who use mobile phones in infancy, I think they will be even more attached to it."
Their results might not that surprising to hear - if you've run out of battery or lost your phone for just a few minutes, you'll know it can get quite stressful.
The fear of being apart from your phone has even got a name - it's called nomophobia, which is an abbreviation of "no-mobile-phone phobia".
It affects about four in five young people, according to some studies.