After playing some more the Binding of Isaac I've come across strange messages that are triggered by certain events.
For instance, when I defeated one of the bosses, I suddenly got a pop-up message that read something along the lines of: "A radioactive spider has appeared in the dungeon" (there was also a drawing of a spider on that message).
Another time, after taking a yellow pill, I got a rather enigmatic hint that said "bombs are keys". I then proceeded to blow up a strange looking rock, only to be greeted with another pop-up saying: "A small rock has appeared in the basement" (this time with a picture of a block. I don't know if taking the pill was actually a prerequisite to get this message).
What do these messages mean, exactly? What am I supposed to do once they appear?