What strange occurrence happens at the zoo

Harry is having a great time at the zoo, until they visit the reptile house. Harry looks at a large boa constrictor encased in glass and feels sympathetic that the snake must suffer through annoying visits from people like Dudley and Piers. The snake winks at Harry, and the two begin a sort of mimed conversation about the snake’s breeding and background: the snake is from Brazil, but it has been bred in captivity. Noticing the snake’s bizarre behavior, Dudley runs over to the glass case, pushing Harry hard to floor in the process. Suddenly, the glass in front of the snake’s cage vanishes and the snake slithers out of the reptile house, playfully nipping at Dudley and Piers as it passes.