There has been a rise in the number of butterfly knives being brought into Guernsey.
These are blades that are hidden by a handle that splits down the middle to reveal the weapon.
Islanders are breaking the law if they buy one without a licence.
According to the police, the popularity of the knives on social media is behind the increased sales.
It does not matter what material the blade is made from.
Therefore, even those made of plastic might be illegal without a licence.
Anyone who is considering buying martial arts or live-action memorabilia should check with the Guernsey Border Agency before ordering in case they unwittingly break the law.