Hell, I'm sure you could arrange for a demonstration: get 5 people to line up. Person one (we'll call him Adam) hands a box wrapped in paper that says "To Bob" to the next person who rips off that layer of paper to show another layer "To Charlie" and hands it to the next person and so on. Then point out that the feds can't see what's in the box, and if the feds look at the box between Charlie and David they can only see that the box came from Charlie and is going to David, but they can't see that it came from Adam or is going to Emily.
Run it again with a cop standing in for Bob and David and show that by comparing notes, Bob knows they got a box from Adam and gave it to Charlie, and David knows he got a box from Charlie at the same time and gave it to Emily. Point out that Adam doesn't know who any of the people are, their computer picks people at random and sometimes the cops get lucky.
<strike>Then point out that if you're the NSA you can use PRISM to track down everyone who handled that box</strike>