What does a numbered black cell mean?
It tells you how many bulbs touch that cell orthogonally.
Can bulbs light through black cells?
No. Black cells block light in all directions.
Do all white cells need a bulb?
No, a cell just needs to be illuminated by at least one bulb.
Can two bulbs be in the same row or column?
Only if a black cell sits between them.
What is the hardest part of Light Up?
Balancing bulb coverage without creating illegal mutual illumination.
