Transcription: In nuclear fusion, two smaller nuclei join to become one larger nucleus. For example, here I have two isotopes of hydrogen. Hydrogen 1, which just has a proton, and hydrogen 2, which has a proton and a neutron. If they collide just right, they can join together to form a nucleus that has two protons and one neutron. That's helium 3. The other thing produced here is a lot of energy, and this is one of the fusion reactions that provides energy for the sun.