Some people including teenagers have joined a group calling itself Islamic State or IS.
It's an extremist Islamist group, based mainly in Iraq and Syria.
They are known for their cruelty.
They have been trying to get people from the UK to join IS in Syria and Iraq by using social media and direct messaging online.
They target vulnerable young people who might be interested in what the Islamic State means.
They promise adventure and fame, love, a perfect home, even their favourite chocolate.
But Islamic State doesn't tell the truth.
It makes promises that it knows are not true.
This is called propaganda.
Instead of travelling to what has been described as an exciting place, the young people find themselves in the middle of a brutal war.
Girls are treated like slaves.
Conditions are bad, and Islamic State refuses to let people leave.
So, even if people change their minds, there is no way back.