Blacken some cells in the grid such that the numbers to the left of (and above) the main grid represent the lengths of contiguous blackened cell blocks in the corresponding row (or column). The lengths are given in order from left to right (or top to bottom), and cell blocks must contain at least one unblackened cell between them. As a special case, if the single clue “0” is given, it means there should be no blackened cells in that row (or column).
(Rules and example from PGP IB)
It is common for a large Paint by Numbers puzzle to feature thick lines every five cells, so that counting black cells becomes easier.
History of the puzzle
Invented independently by Tetsuya Nishio (Japan) and Non Ishida (Japan) in 1988. The first magazine issue they respectively presented their first puzzles were, coincidentally, both published on July 2nd, 1988.
Cross the Streams
See Cross the Streams.