Complex sentence: Napalm is a thickening/gelling agent generally mixed with petroleum or a similar fuel for use in an incendiary device, initially against buildings and later primarily as an anti-personnel weapon that sticks to skin and causes severe burns when on fire.
Split into simpler sentences: Napalm is a mixture of a thickening/gelling agent and petroleum or a similar fuel for use in an incendiary device. Initially used against buildings and later primarily as an anti-personnel weapon that sticks to skin and causes severe burns when on fire.