Several police forces are warning parents about a social media "game" which has reportedly been linked to teenage deaths in Russia.
The blue whale challenge is reported to involve people being set a series of tasks over 50 days - ending in the person taking their own life.
There's no evidence the game has reached the UK, or whether it's real.
Newsbeat has spoken to a number of police forces who say they're aware of the challenge.
In it, individuals are given tasks by an anonymous "master".
These are reported to escalate from straightforward demands, like watching a scary film, and become increasingly dangerous.
Hertfordshire Constabulary is advising parents "to be vigilant and monitor children's internet usage".
Woodlands School in Basildon, Essex, has sent parents a letter explaining the risks involved with the game with the aim of stopping it from happening.
"There's no evidence to say anything untoward is going on with the blue whale challenge in our school, but we feel it's best to keep parents and carers informed," deputy head David Wright has explained to Newsbeat.
"We [want to] make sure they can help us. We've all got a duty of care for our young people."
Unconfirmed Russian media reports have linked the deaths of two teenage girls to the challenge.
The name of the game appears to be a reference to the way some blue whales beach themselves on purpose and die.
Bulgaria's Centre for Safe Internet has told the Balkan Insight that the game is most likely a rumour which has been spread online.
It says there's no concrete evidence for the existence of the blue whale challenge.
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