"That's... a really good idea, actually."
She'd experimented with this particular stone already. As she'd pointed out, the cave had been in use for some years now. Fading scratches on the wall revealed previous attempts at, well, copying the Destiny Island cave, to put it bluntly.

She plucked a palm-sized stone from a small pile that looked out of place in the otherwise barren floor. This she used to score a thin, faint mark in the wall as close to the edges of the window as she could get it without sticking her hand in.

Afterwards, she threw the rock at the window and watched it bounce off, skipping back a bit to avoid having a toe smashed.