Masyu

[from nikoli website]

1. Make a single loop. Lines must pass through the centers of cells horizontally or vertically and never cross, branch off, or go through the same cells twice.
2. Lines must pass through all cells containing black and white circles.
3. Lines passing through a white circle cell must go straight through the cell, then make a right-angled turn in the very next cell (on at least one side of the white circle cell).
4. Lines passing through a black circle cell must make a right angled turn immediately, in the black circle cell, then go straight for the next two cells.

Solution: