LHCb is part of the International Masterclass Programme: each year, high-school students reach their nearby universities and become LHCb physicists for a day. Their goal is to select in real LHCb events a particular decay of the D0 meson, where a Kaon and a Pion are produced. On such events, the D0 lifetime is measured, using the same analysis tools and techniques LHCb researchers use every day.