Here's why:

* Uranium is the heaviest naturally occurring element. Fermi reasoned that adding a neutron to uranium's nucleus would create an element with a higher atomic number, pushing it beyond uranium on the periodic table.

* Transuranic elements were unknown at the time. The idea of elements heavier than uranium was purely theoretical. Fermi's experiments were designed to test this possibility.

It's important to note that Fermi's initial interpretation was incorrect. While his experiments did indeed produce nuclear reactions, the resulting elements weren't transuranic. Instead, they were fission products, meaning the uranium nucleus split into lighter elements. This discovery, later made by Hahn and Strassmann, revolutionized our understanding of nuclear physics and led to the development of nuclear weapons and energy.