Usually I show current activity in my blog posts, but this is a bridge I designed and built a few years ago, from Eitech parts. It illustrates the use of adjustable length parts to solve a design problem, where using a standard length part would have severely disrupted the curve of the arch. Adjustable length parts in construction sets have been around at least since Richter's Anker bridge set of 1895, and are still a regular component in sets from Eitech and Merkur and perhaps others.
But in this bridge, as in the Engino bridge I built yesterday, I find the adjustable parts are not entirely satisfying.
First, the parts represent bridge members that would be under compression (forces push on the ends - the member must not collapse), but the parts have the appearance of tension members (forces pull on the ends - the load might instead have been handled by a cable). When you look at the tower crane in an image from the Engino box, the upper, diagonal usage of the expansion parts looks reasonable - a cable would work here; while the lower, vertical usage as a kingpost is more doubtful, though not as bad as the bridge. In the adjacent suspension cable car (have I identified this correctly?), the parts look fine to me - such vehicles typically use lightweight components where they can.
Second, I am not completely comfortable with the use of adjustable "building blocks." A good portion of what a construction set should provide is exercise in solving problems with standard elements. Deviating from this, whether by adjustable parts such as this or the provision of new ad hoc parts designs (Lego is especially egregious), reduces the "puzzle pleasure" and may also reduce the learning value.
If a child is playing with a set that has only limited positions for fastening parts together, meaning that only certain choices for two sides of a triangle can provide for an appropriate third side allowing for a right triangle, he or she may not rediscover the Pythagorean Theorem, but they will at least have some context for recognizing that the Pythagorean Theorem relates to the real world. Will they get this, if they have solved all their triangle problems by simply readjusting the expandable part?
Further, are we then teaching them how to create and adapt and problem solve within constraints, or are we teaching them that the world will adjust itself to their needs and expectations?
Block Play, at whatever level of literalness or metaphor, is importantly an exercise of balancing constraints with versatility.
A construction set, if it is to be both enjoyable and educational, needs to balance those demands of constraint and versatility. Each constraint can have both pros and cons, and each versatility can have both pros and cons.
The Eitech sets, the Merkur sets, and the Engino sets, with or without the expandable parts, provide a variety of trade offs of constraint and versatility. Each, I believe, provides more pros than cons, and each is a worthy candidate for good Block Play.