You may have seen Foucault's pendulum. There's one in the Science Museum in London (part of the National Museum of Science and Industry), and there are many more in various locations around the UK (for instance, in Glasgow) and the world (including one at the United Nations Headquarters and a famous example at Le Panthéon in Paris).
From September 1999 onwards, PASS Maths is joining forces with its sister site NRICH to offer many more puzzles changed regularly on the 1st of every month.