Region puzzles
Cut the grid into pieces, or fill the pieces it already has. Shikaku divides the grid into rectangles whose areas match the numbers; Aquarium fills pre-drawn tanks with water that must sit level. In both, the region borders are the clue, and the trick is to see which cells a region can and cannot include.
All region puzzles
What makes a region puzzle
A region puzzle is about areas rather than single cells. In Shikaku every number is the area of exactly one rectangle that contains it, and the rectangles tile the grid with nothing left over. In Aquarium the regions are given and you decide the water level of each one: water fills a tank from the bottom up, so every filled cell has filled cells beneath it in the same tank, and the numbers outside the grid count filled cells per row and column.
Start by asking which cells each region can reach. A 2 in Shikaku has at most four rectangles, and usually only one that does not collide with a neighbour. In Aquarium, a row count of zero empties a whole stripe of every tank it touches, and a full row fills them. Shikaku is the gentlest puzzle on the site; Aquarium is a good second step.
FAQ
No. Every rectangle contains exactly one number, and that number equals the rectangle's area. Squares count as rectangles.
Each tank has one water level. All cells of a tank at or below that level are filled and all cells above it are empty, so the water is level within a tank even when the tank has an odd shape.
No. Every grid here is verified to have a single solution reachable by logic. If you are stuck, use Check to confirm what you have drawn so far is consistent.