Building the shortest bridge is easy, but to build the medium length one visitors need to balance the bridge they are building out into the river by extending their structure out on the shore side too, and then stacking extra blocks to balance those that stick out furthest. Once the medium sized bridge is built, spanning the widest section of river is a matter of scaling up the same principles, which encourages builders to really think about what they are doing.
This is a classic exhibit in the sense that it's seen in lots of places. More importantly, it's a classic because it's very simple but allows lots of space for exploration and challenge. At first glance it looks like a 'puzzle' exhibit with a 'right answer' (a class of exhibit that from my point of view has a number of down sides), but this one is different because in fact there are many ways to successfully bridge over the river, and a challenge to minimise the number of blocks used is a really successful way to get visitors exploring their options more fully and (often without noticing what they are doing) experimenting with both the physics of moments and the engineering of "good enough".