No. 1
Anne and Jessica were working on the computer along with their friends Sandy and Nicole. Suddenly, I heard a crash and then lots of shouts. I rushed in to find out what was going on, finding the computer monitor on the ground, surrounded with broken glass!
Jessica saying, "It wasn't me!"
Sandy saying, "It was Nicole!"
Anne yelled, "No, it was Sandy!"
With a straight face Nicole said, "Sandy's a liar."
Only one of them was telling the truth, so who knocked over the monitor?
It occurs to me that No. 1. can serve as the basis for a further puzzle, if, instead of having only one girl telling the truth, posing the question of whodunnit when only one of the girls is lying.