Admitting you don't know

Ni idea

nee ee-DEH-ah — No idea

"¿A qué hora empieza? — Ni idea."
What time does it start? — No idea.

Shorter and more casual than "no tengo ni idea" (the full version) — perfectly fine to use on its own among friends.

"The honest answer I give at least once a lesson, whenever a student asks me some very specific grammar exception."— Graciela