Maths Challenge
Maths challenge.net: Spider Fly Distance
A "three star geometry problem" requiring knowledge of how to complete geometric proofs and some prior math skills to figure the shortest distance between the opposite corners of a cuboid.
Wolfram Research
Wolfram Math World: Distance
MathWorld.com gives a formula for finding distance using the Pythagorean theorem. The formula for finding the distance between two points in Euclidean space is given on this page. The page has links to related topics.