How to solve Light Up
Start at the numbers. Fill the ones with no spare sides, dot the ones that are done, then light each dark cell from the only place that can reach it.
How it works
Light Up gives you a grid of white cells and black cells. You place bulbs in white cells. A bulb lights its row and column in both directions until a black cell stops the light.
Three rules must all hold at the end. Every white cell is lit. No bulb shines on another bulb. A number on a black cell counts the bulbs on its four sides, exactly.
Three moves solve most grids, and you repeat them. One: a number with exactly as many open sides as it needs gets a bulb on every open side. Two: a number that already has its bulbs gets a dot on every other side. Three: a dark cell that only one open cell can light gets its bulb there. A lit cell never counts as open, because a bulb there would shine on the bulb that lights it.
Where to look first
Numbers against the edge or beside other black cells have fewer sides, so they fill up first. A 4 always takes all four sides. A 0 dots all of them.
After every bulb, look along its row and column. Cells it lights are no longer open, and a number beside them may now have no spare sides.
Dots are worth placing
A dot says “no bulb here”. It doesn’t light anything, but it takes a cell out of the count for its numbers and for the dark cells around it. Most solves stall because a dot was not placed.
Worth memorizing
Open sides equal the number: fill them all.
Lit sides don’t count as open.
A number with its bulbs: dot the rest.
A 0 is done from the start.
A dark cell with one possible light: put the bulb there.
The bulb can be the cell itself.
Practice
0 of 4 rightOn the empty grid, which number already tells you where every one of its bulbs goes?
Can the ringed cell hold a bulb?
What can light the ringed cell?
Seven bulbs are in. How many white cells are still dark?