What is the puzzle similar to sudoku where you have fill in numbers so they are all connected?

The puzzle you're thinking of is called "Slitherlink".

How to Play Slitherlink:

1. Grid: The puzzle starts with a grid of squares, usually 9x9 or 10x10.

2. Numbers: Some squares contain numbers from 1 to 3 (or sometimes higher). These numbers represent how many sides of the square will be part of the continuous loop.

3. Loop: Your goal is to draw a single, continuous loop by connecting the sides of squares.

4. Connecting: The loop can only go horizontally or vertically, never diagonally.

5. Numbers Rule: The number in a square tells you how many sides of that square will be part of the loop.

Example:

If a square has the number '2' in it, you must draw a line on two of its sides. If it has the number '1', you draw a line on one of its sides, and if it has a '0', you don't draw a line on any of its sides.

The Challenge: You need to figure out the correct placement of the loop lines so that all the numbered squares have the correct number of sides connected to the loop, and the loop is a single, continuous line with no breaks or intersections.

Let me know if you'd like to try a Slitherlink puzzle!