Jack and Jill happens to want to make a sandwich at the same time. Both need a slice of bread, so they both goes to get the loaf of bread and a knife.
Jack gets the knife first, while Jill gets the loaf of bread first. Now Jack tries to find the loaf of bread and Jill tries to find the knife, but both find that what they need to finish the task is already in use. If they both decide to wait until what they need is no longer in use, they will wait for each other forever. Deadlock.