Toys that come to life, mythical creatures or simply people like their peers — imaginary friends are dreamed up by children the world over.
For some it will be a short invention, for others it can endure for several years, according to clinical psychologist Lisa Ford.
Children invented playmates so they could experiment social skills and set new challenges for themselves, she said, adding that it was more common in sole children or the oldest child.
"A lot of parents are naturally quite concerned by it, but it's quite a common phenomenon," she told Paul McIntyre on ABC Radio Hobart.
"About two-thirds of children will invent an imaginary friend at some point in their development.
"It's really safe to [practice social skills] with an imaginary play partner because they won't receive as many challenges back."
A healthy phase
Dr Ford said make-believe friends were a healthy part of childhood.
"Once upon a time, it was assumed that having an imaginary friend reflected some sort of social difficulty or that a child was really struggling developmentally or emotionally," she said.
"But we know from research that it's not the case — children with imaginary friends are less shy and have better perceptive skills."
It also boosted their language skills, she said.
The imaginary friends might be personified toys, animals, mythical creatures or even whole worlds or groups of friends.
"As parents we can sit back and relax and enjoy it — it can be really quite insightful watching your child interact with their friends and the conversations that come up.
"I think it's a nice stage of a child's development."
Dr Ford said the friends appeared between the ages of three and seven and could foster quite vivid interactions.
"They are life experiences they are having with these friends," she said.
"Interestingly, the children will tell you, most of the time, that they know the friend isn't real.
"But the actual experiences they are having feel really real."
Remembering special friends
Many of ABC Hobart's audience have fond memories of their make-believe childhood companions.
"Sebastian, he was a dragon. I used to go flying with him." — Clancy
"My daughter had an imaginary friend called Robot!" — Sue
"Tracey, I miss her. The best listener and friend. No complaints about what we did or where we went." — Kelly
"Evie, inspired from the TV show Out Of This World. We threw her in the bin once when we had a fight and I went crying to mum, who had to get her out of the bin." — Leah
"Our son had an imaginary little dog called Runny, who he said he brought home from Bruny Island." — Chris
"A dolphin and a horse, but they had no names." — Rachel
"My brother had imaginary friends called Choog and Wigg." — Emma
"I used to have my imaginary rainbow flying unicorn." — Jasmine