Azulejo is a form of a painted, tin glazed, ceramic tile.
Constantly in production for the past five centuries, they are a huge aspect of Portuguese culture and you can expect to find them in churches, palaces, the inside and outside of people's homes and even train stations and the underground!
They are applied to walls, floors and ceilings and were not only for decorative use, but also to control temperature in homes.
Azulejo comes from the Arabic word 'Zellige', which means - polished stone. It was introduced to Portugal (via Spain) by the Moors, the Spanish city Seville being the main centre for the tile industry.
So should you ever have a day like ours today, when the heavens well and truly open and it throws it down, then the museum in Lisbon is worth a visit. Plus on Sundays most of the museums are free!