Porto is Portugal's second-biggest city, built on a hill above a wide river called the Douro. On the night of São João, families set up little grills outside their houses, cooking sardines and corn for anyone walking past. The smell drifts everywhere. People put up small flags and balloons in their windows.
One of the strangest traditions is the plastic hammer. Children and grown-ups buy small squeaky plastic hammers from street stalls and bop each other gently on the head with them as they walk past, saying 'São João!'. It is just for fun - everyone does it, and everyone laughs. Older traditions used a leek for the same job.
All night long, music plays in different squares: pop, traditional Portuguese folk, brass bands. People dance until very late. At midnight, the sky over the Douro river fills with fireworks, with thousands of people lining the bridges to watch.
Later in the night, when the fireworks have stopped, hundreds of glowing paper lanterns are released into the sky. Each one floats up like a tiny hot-air balloon, and the night fills with little flickering lights drifting slowly out over the river. It is one of the most beautiful sights in Portugal.