Some schemes are so very logical that they teeter on the edge of reason. Take the 2001 project "Pig City" by the highly-regarded Dutch architects MVRDV - who have been selected to design this year's Serpentine Gallery pavilion in London. The architects noticed that there are almost exactly as many pigs as humans in the Netherlands. Rather than reclaim yet more land from the sea to put them on, MVRDV suggested a high-rise Utopian metropolis of vertically-stacked pig farms, echoing Le Corbusier's plans of the 1930s. The towers would have lagoons at their base to breed fish for the pigs' protein feed. Each self-sufficient tower would contain its own slaughterhouse; each tower would be capped by a methane-collecting dome that would power the complex from pig manure. There are even projecting field-decks, high in the sky, to allow the pigs to roam in the open air. The whole concept is entirely brilliant and practicable. But it raises awkward questions: elevated in this way, are the pigs becoming more human? Are we becoming more like pigs? Are we just too close for comfort?

A number of the schemes presented play games with the viewer. Some are obviously done for a laugh, such as the "Good Old Days of Architecture" monument of 1951 by Thomas Greeves, complete with Tudorbethan helicopter: pure Festival of Britain whimsy. Others have a kernel of seriousness beneath the fun. I still don't really know how tongue-in-cheek were the giant shrimp-buildings proposed for Morecambe Bay by architects Birds Portchmouth Russum in 1991. The idea was that Morecambe would have landmarks to rival nearby Blackpool. The shrimp-buildings were assigned functions. The draughtsmanship, as with all BPR's work, is extraordinarily fine. Their later scheme to turn the multi-storey car parks of Croydon into a series of entertainment venues was lateral thinking of a high order, and was taken seriously by at least some of the local politicians of the time.