Draw some horizontal and vertical lines so that all the circles are connected (directly or through connections through other circles). No two circles may be directly connected by more than two lines. Lines may not cross other lines. If a circle is numbered, then that number represents the number of lines that are connected to that circle.
(Rules from PGP IB and example from WPC 2018 IB)
History of the puzzle
First appeared on Nikoli volume 31 (1990). Invented by れーにん ("Renin"). Originally named 橋をかけろ ("Lay bridges").