The Franklin Institute is one of the oldest science museums in the country — founded in 1824 to honor Benjamin Franklin — and one of the best. The giant walk-through heart is a Philadelphia institution: a two-story model of the human heart that visitors walk through while listening to the amplified sound of a heartbeat. Children sprint through it; adults walk more slowly and feel oddly moved. It was installed in 1954 and has been the museum's signature exhibit ever since.
The Benjamin Franklin National Memorial, at the heart of the building, is a 30-foot marble statue of Franklin sitting in a classical rotunda modeled on the Pantheon. It is genuinely imposing. The rest of the museum covers everything from space exploration to electricity to the physics of trains, with hands-on exhibits that reward both curious children and adults who have been wondering how things work. The planetarium is excellent; the IMAX theater runs a solid program. Allow at least three hours if you want to see everything.