An obelisk is a tall, four-sided pillar of stone that comes to a point at the top called a pyramid. The Egyptians carved obelisks from single pieces of granite - an extremely hard rock - and covered them with carved pictures called hieroglyphs. This particular obelisk is about 25 metres tall and was originally quarried and shaped in Egypt, probably around 1,800 BCE.
The Romans brought it to Rome by ship across the Mediterranean Sea, which was an extraordinary engineering challenge. Obelisks are extremely heavy - this one weighs around 330 tonnes - and moving one without modern cranes or lorries required enormous skill and hundreds of workers. Once it reached Rome, it stood in a stadium for many centuries.
In 1586, workers moved the obelisk from its old location in Rome to the centre of St Peter's Square - again without any modern machinery. It took hundreds of men, dozens of horses, and many specially built wooden machines to lift the obelisk upright and set it on its stone base. According to the story, the crowd was completely silent during the whole operation, and when it was finally in place, everyone cheered.