A PET scan is a method of measuring the level of activity in a certain brain region during a mental task. If we wanted to measure what happens when someone speaks we would first 1) inject a small amount of harmless radioactive material with sugar into the brain which will emit particles that can be measured by a PET scanner, 2) have the patient speak about what is on their mind while their head is in the PET machine 3) examine the printout which will show that the regions with the most activity used the most sugar and since the sugar was connected to the radioactive material we can deduce that the most particles were emitted from the areas that were most active.