what happens when pennywise opens his mouth

When Pennywise opens his mouth in Stephen King’s IT universe, the clown’s face splits unnaturally wide to reveal rows of monstrous teeth and, most disturbingly, three glowing lights known as the Deadlights deep inside its throat. These Deadlights are essentially the creature’s true, otherworldly essence, and direct exposure overwhelms a victim’s mind, often putting them into a catatonic “trance” and, with long enough exposure, driving them insane or destroying them mentally.

On-screen moment

In the modern IT films, this is shown most clearly when Pennywise is about to consume Beverly Marsh:

• His jaw opens far beyond human limits, the facial red lines marking how the mouth peels back.
• Three bright, rotating lights appear at the back of the throat, and Beverly suddenly “floats” mentally, frozen and entranced rather than immediately killed.

What the Deadlights do

Within the lore:

• The Deadlights exist “outside” normal space and time and represent IT’s core life-force, not just a physical trick.

• Looking directly into them can render a person unconscious almost instantly, and prolonged exposure can cause permanent insanity or leave them effectively broken beyond recovery.

Why Pennywise uses this form

Pennywise usually relies on fear, claws, and teeth, but opening its mouth to reveal the Deadlights is a kind of last-resort weapon:

• It tends to be used on characters who resist fear or show unusually strong will, such as Beverly or other determined victims.

• By forcing them into a helpless, floating trance, Pennywise can then toy with or consume them at leisure, turning courage into vulnerability.

TL;DR: When Pennywise opens his mouth fully, you are seeing IT pull back the clown “mask” to expose its true essence—the Deadlights—which can hypnotize, break, or mentally destroy anyone who stares into them.