0 moves · par 0
N for a new building · R to play this one again
Every puzzle is made by pressing a random set of windows on a dark building, so it can always be undone. Par is the true minimum number of presses, found by Gaussian elimination over GF(2); on 5×5 four different presses cancel out in combination, so there are sixteen solutions and par is the lightest one.
seed 0 · size 5. The default seed is today's date, so each day has its own building. ?seed= and ?size=5|7 set both from the URL.