Extensor pieces use a scissor-like mechanism to extend and contract, and can be connected together to build shapes and structures which can then themselves expand and contract in cool ways.
This starter set contains 40 plastic parts (including X and Y pieces for making more interesting shapes), two springs and an instruction booklet. Ages 13 and up.
Price = (the 367th prime number) pence = 2477 pence = £24.77