How does uranium turn into lead?

Uranium is radioactive. Which means its nucleus will emitt an alpha particle (two protons and two neutrons) spontaneously. Because the nucleus lost two protons it becomes the element Thorium. Thorium also emitts alpha's and changes to Radium. This process continues; Radium into Radon into Polonium and finally into lead. The final Lead is not radioactive and the process ends. The actual process is a little more complicated because some of these intermediate elements can change by converting a neutron into a proton and emitting an electron (beta radiation), but the basic process is one radioactive element changes into another radioactive element by emitting radiation (alpha's or beta's). The Uranium to Lead Process has a half-life of about 4.5 billion years. Meaning that in 4.5 billion years 1 kilogram of Uranium will have changed to a half kilogram lead and a half kilogram Uranium is still left. This is approximate because there will also be some of those intermediate elements waiting to change.