Certain blocks (like half of a tree, or half of a house in a village) appear invisible. You cannot walk through them, or fly through them, almost as if there are blocks there, accept you see a wiggle effect similar to being stuck inside a block. When you place a block where this effect takes place, it is instantly replaced by whatever block is supposed to be there. Example: in a village, you see half a house, you observe that you cannot fly/walk through the empty half of the house, so you place a grass block where the problem occurs and it instantly turns into cobblestone, glass pane, wood, etc. whatever the house was supposed to be made out of.