Niels Bohr was a Danish physicist who developed groundbreaking ideas about how atoms are structured, explaining how tiny particles inside them behave in strange, unexpected ways. His work helped launch the field of quantum physics, one of the biggest scientific breakthroughs of the 20th century, and won him the Nobel Prize in Physics in 1922. During World War II he helped fellow scientists escape Nazi-occupied Europe before fleeing Denmark himself in 1943. He also mentored many other important scientists and later worked to promote the peaceful use of atomic energy.