In 2004, some very bad people decided to hurt a lot of innocent people in Madrid, Spain. They did this by putting bombs on trains that were carrying people to work or school.
When the bombs exploded, lots of people were hurt and many died. It was a scary and sad day for many people who lived in Madrid and around the world.
The people who did this did not want to be nice, they wanted to be mean and hurt other people. This is called terrorism and it is very wrong.
Thankfully, the people who did this were caught and punished for their terrible actions. But it is important to remember the innocent people who were hurt and their families who still miss them. We should always be kind and not hurt others because hurting people is never okay.