Melissa 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! Sandy and Jessica spoke almost at the same time:
Jessica saying, "It wasn't me!" Sandy saying, "It was Nicole!" Melissa yelled, "No, it was Sandy!" With a pretty straight face Nicole said, "Sandy's a liar."
Only one of them was telling the truth, so who knocked over the monitor?