Puzzle in plain Geometry

This is my favorite puzzle. It has the rare quality, that it only demands the most basic knowledge in plain geometry, but most people find it extremely difficult to solve.

To solve the puzzle, you will need the AHA-insight flashing experience.

When presented with the solution you will find it obvious, and you will understand it immediately.

I will not present the solution to you. It is hidden on this website. But if you want it, then send me an e-mail.

Here is the puzzle: 

In a square width sides of length s, choose any point P. From P, draw 3 straight lines to corners of the square. The lines are called a, b and c, with b being the one in the middle. It can be shown - and the solution will show it clearly - that these 3 lines uniquely determines the length of the side of the square.

So now the problem is:

Given the 3 lines a, b and c, construct by means of ruler and compass the side s of the square.