If you carry an offensive weapon, like a knife, in a public place, you can be arrested by the police.
It's important to know that if you use a knife to hurt or stab someone then it's very likely you will end up in a young offender's institute.
The punishment you'll get for carrying a knife depends on what you were carrying, why you were carrying it and how many times you've done it before.
You could be given a police caution, which is a serious telling off, or depending on where you live, a more severe punishment...
In England and Wales...
The government and police take knife crime so seriously that in 2007 they increased the maximum prison sentence for carrying a knife from two years to four.
If you're aged 14 to 16 you could be sent to a young offenders institution or secure unit.
Children, aged 10 to 14 are more likely to have to spend time with a social worker every week rather than being locked up.
In Northern Ireland
The maximum sentence in Northern Ireland was also increased in 2007, from two years to four.
Sentences for children are being reviewed and could be changed after spring 2008.
But, at the moment, if you are aged 10 to 17 and found guilty of carrying a knife you could be sent to a criminal justice centre.
The maximum prison sentence is four years, but if you're under 16, it's more likely your case will be referred to someone called a Children's Reporter.
They investigate your case and decide if it needs to be sent to a children's hearing.
If it does, a panel of trained volunteers from the local community decide what your punishment should be.
They might make a supervision order, which means you can stay at home but have to do certain things - like be home by a certain time every day or attend special behavioural sessions.
Or they could send you to a secure unit for a while.
In the Republic of Ireland...
Children under the age of 12 cannot be charged with an offence, unless it is something really serious, like murder.
But that doesn't mean you'd get away with carrying a knife as the Gardai (or police) would have to tell your parents, or whoever looks after you, about what you've done.
You may also have to go to special sessions with a social worker to help you with your behaviour.
Adults caught carrying a knife face a fine of up to £1,000 and five years in prison.