A psychologist has come up with a novel idea for helping parents find out the perfect Christmas toy for their children - a mathematical formula.
Dr Cliff Arnall, who has previously come up with calculations for the happiest and most depressing day of the year, reckons that mathematics can be used to work out the "play value" of potential purchases.
But the maths is nowhere easy to solve, and the result is also likely to be different for different children.
Parents first have to assess the toy against six criteria: its usefulness when playing alone (Pi); playing with others (Po); whether it fosters creativity (Cr); its value promoting social interaction (S); its general utility (U) and finally whether it can be handed down to siblings (H).
Parents must give the toy a score out of five for each and add them up.
In a separate sum, then they must estimate the amount of time in hours a child will play with the toy regularly (T) and multiply it by how many months it is likely to remain of interest (L).
The two figures must then be divided by the square root of the cost (c) and added to the total score from the criteria section.
Simple? For those who did not quite follow, it works out as: T x L + Pi + Po + Cr + S + U + H / square root of C
"A toy may score higher for one child than another but that's an important part of the exercise," the Telegraph quoted Dr Arnall as saying.