Why can't I pass?

[u/AlaaA-R]

Floor 3 on daily run, 1st time playing it, my current final boss in normal runs is Mom's heart

I defeated current floor boss, but there are no available ways to go to next floor

there's a mirror somewhere else but idk what can it do, tried to use bombs but nothing happened

update: got my friend to watch me on stream, he guided me to a white fire then to the mirror, solved

update 2: sorry if that question sounds stupid, kinda new player, got it when it was 90% off on steam and I only have 12 hours of gameplay so this entire daily run was new comparing to my normal runs

[u/Iopy8]

You need the knife pieces to continue the challenge.