Puck gives Bottom the head of an ass because he thinks Bottom has been acting like an ass. Puck has been watching Bottom and the other tradesmen practice their play they hope to perform for Theseus's and Hippolyta's wedding celebration. During the practice, Bottom shows himself to be less than intelligent, boastful, and essentially a swaggering fool. Since Puck tends to be ornery, he decides that Bottom should look like an ass since he is acting like one. Bottom is unaware of the transformation so when Snout says, "O Bottom, thou art changed. What do I see on thee?" Bottom replies, "What do you see? You see an ass-head of your own, do you?" and then when Quince also exclaims over Bottom's transformation, Bottom again ironically says, "...This is to make an ass of me, ...". He thinks the players are just trying to make him feel foolish and he continually uses the word "ass" ironically.