The London Stone is a big old rock in the city of London. A long time ago, people thought it was really important because it marked the spot where all the streets and roads in the city began. It was even more important because it was near a river called the Thames, which was like a big highway for boats that were bringing goods to the city.
Now, the London Stone is what we call a "riparian" object. "Riparian" means something that is connected to or located near a river. The London Stone is riparian because it's near the Thames, and that river was really important for the people who used to live in London a long time ago.
People used to say that the London Stone was really magical, and that it had special powers that would help protect the city. They wanted to make sure that the rock was always there, so they built things around it to keep it safe. Nowadays, the London Stone is still there, but it's in a fancy case in the middle of a building so that it doesn't get damaged or lost. People still think it's important because it's one of the oldest things in London, and it reminds us of the city's history.