Cosmic rays are charged particles that fly to us from the depths of the Galaxy. If you plot how many particles arrive at each energy, the line runs smooth, then suddenly drops sharply. Such bends are called 'knees.' They show that some cosmic particle accelerator has reached its limit and cannot boost particles any higher.

The NUCLEON instrument on a satellite spent several years collecting particles and measuring their energy in two different ways, like using two rulers. Both methods showed the same thing: a new knee at a rigidity of about 10 teraelectronvolts. Moreover, this knee is common to all nuclei — from protons to iron. Previously, it was thought that heavy nuclei break later than light ones, but here they all break at the same place.

What is this limit? Most likely, some source of cosmic rays, perhaps a supernova remnant or stellar wind, stops accelerating particles above this mark. If there is one source for all, then the knee is common. It's like if the water in all the pots in a kitchen stopped heating above a certain temperature: it means the stove is the same one.