In order to initiate fission, a high-energy neutron is directed towards a nucleus, such as 235U. The combination of these two produces 236U, which is an unstable element that undergoes fission. The resulting fission process often releases additional neutrons, which can go on to initiate other 235U atoms, forming a chain reaction. While nuclear fission can occur without this neutron bombardment, in what would be termed spontaneous fission, this is a rare occurrence; most fission reactions, especially those utilized for energy and weaponry, occur via neutron bombardment. If an element can be induced to undergo fission via neutron bombardment, it is said to be fissile.
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