The solar neutrino problem is like a mystery that scientists had to solve. It all starts with the sun. The sun is made up of tiny little particles called neutrinos. Scientists wanted to know how many neutrinos were being given off by the sun, so they ran experiments to try and measure how many neutrinos were being made by the sun.
When they looked at their results, they found that there were much fewer neutrinos being given off than they expected. This was a big problem because scientists had to figure out where all the extra neutrinos were going, or why there were fewer neutrinos than expected. Scientists all over the world worked together for a few decades to solve the mystery and eventually figured out that the neutrinos were changing from one type to another as they travelled from the sun to the Earth!