The film opens with unquestionably one of its most shocking moments, as young Georgie sails his paper boat down the street, only for it get washed into the sewer.
At that moment, Pennywise makes his first appearance, holding Georgie's sailboat and seductively introducing himself to an uneasy Georgie.
The scene ends with Georgie reaching into the sewer for the boat, at which point Pennywise bites down on his arm and rips it clean off. Poor Georgie tries to crawl away, but Pennywise extends his arm out of the sewer and pulls him in. Roll titles.
It perfectly sets the mood for the rest of the film and is certainly one of the most brutal moments of any major studio horror film in recent years.