Britain is surrounded by sea. Warm, wet air blows in from the Atlantic Ocean to the west, where the water is warm because of a giant ocean current called the Gulf Stream. When that warm, wet air hits the cooler land, the water in it falls out as rain.
Because Britain is small, the weather can change very quickly. A morning might start sunny, turn cloudy by lunch, rain in the afternoon and be bright again before tea. This is why British people talk about the weather so much - it really does keep doing something different.
The wettest place in the UK is a tiny village called Seathwaite in the Lake District. It gets around 3.5 metres of rain a year - taller than the tallest basketball player ever, lying down. The driest places, in the east of England, get less than a third of that.
Rain is one reason the UK looks so green. Grass loves it. So do trees, sheep, gardens and the famous British lawn. People joke about the weather, but the rain is also why the countryside is the colour you see on biscuit tins and tea boxes.