Surprise needs no disappearance. Although your sister's presence behind a curtain remains certain, her return to view can arrive at a moment your attention has not prepared for. Knowledge is not readiness. The distinction helps explain why an ordinary reveal can produce a flicker of astonishment without requiring any belief that she has left the room.
Certainty has its limits. Object permanence, the understanding that something continues to exist when hidden, supports your confidence that your sister remains nearby, but it does not specify exactly when her face will emerge. Timing changes the experience. If the curtain moves sooner than expected, or her expression differs from what you anticipated, the incoming sight can generate prediction error: a mismatch between an expectation and what actually appears. Nothing supernatural has happened. What feels surprising may therefore concern the manner of her appearance rather than the fact of her existence behind the fabric.
Attention adds another twist. While you are looking elsewhere or thinking about something else, a sudden movement at the curtain can draw your attention back before you consciously assess how predictable the event was. Familiarity offers no immunity. A familiar face can still interrupt a quiet visual scene, although the strength of that response depends on timing, context, and how closely you were watching. The curtain conceals timing. Between knowing who waits behind it and seeing her step into view lies a small interval in which certainty can coexist with surprise.
