I can't find the answer anywhere else and I need it for an essay (I'm in 8th grade). Please answer with as much detail as possible.
Any websites I have found have conflicting answers. One said that they don't actually burn underwater, rather than separate hydrogen from oxygen and burn at the surface. Another actually said that they do burn underwater because they take the oxygen source from the water. I do not have a specific definition for "burn" (the question itself is my assignment- write an essay answering that question!), so I know about as much as you do concerning the wording of the assignment.
I do know that alkali and alkaline earth metals are generally very reactive with water, and usually result in exothermic reactions.