Answer
The answer lies in their number, a phenomenon that baffled scientists as solar neutrino experiments consistently detected far fewer neutrinos than predicted by standard solar models. This discrepancy, known as the solar neutrino problem, captivated Pontecorvo's attention, leading him to propose the concept of neutrino oscillations as a possible solution. His groundbreaking ideas significantly influenced the field of neutrino physics and set the stage for subsequent discoveries.