It's the magic of Lucburg movie science. Basically Lucus and Spielburg take plausible movie science, say, flipping a bike by sticking a rod in the front wheel, give you the finger and turn it into Lucburg movie science. AKA magnetic crystal skulls from aliens that come back to life once said skull is attached.
That is a pretty good explanation. The part where Spalko's eyes exploded made me scream. I was watching it on a flight to London to visit my aunt and it was 2:00 in the morning and the stewardess got mad at me for waking people up. =P
I'm not sure why, but I seemed to like Temple of Doom the most. I loved Last Crusade too, but Raiders was a classic, so that's my second favorite. Kingdom of the Crystal skull Shouldn't even be considered an Indiana Jones movie IMO. These movies are supposed to be realistic, and I know Temple of Doom wasn't, but at least it made sense because they were on a mountain (I think, I haven't seen the movie in a while) and there was a civilization there that probably did those kinds of rituals. But anyway, Crystal Skull was not interesting at all either.