French police are warning young people about the dangers of a dare spreading on Facebook that has left a young man dead and another severely injured.
Police confirmed that a 19-year-old had drowned after riding his bicycle into a port in western Brittany while a friend filmed him.
Another youth was injured for life after diving head-first into shallow water near Calais.
The challenge dares people to jump fully clothed into icy water while friends film it.
"Don't be influenced by a stupid phenomenon of the moment," national police warned on Facebook.
Police spokesman Sylvain Laniel said the bike rider in the town of Beganne had his ankle attached to the bicycle.
Youngsters are challenged to take up the dare - or buy a meal in a restaurant.
Authorities advised young people to "show friends you are more intelligent than that" by refusing the dare, and added that videos of such pranks might one day compromise job applications.
In the US, several people have been injured participating in a Cold Water Challenge on Facebook. The dare is to jump into cold water, or give money to a designated charity.
Casualties have included a 16-year-old girl in Wisconsin who suffered knee ligament damage, and a student in Illinois who fractured an ankle.
In February, officials in Britain said several deaths had been linked to "neknominate", an online dare to drink in excess.