Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Creep usually takes place where the ground freezes and thaws frequently. Soil and rock particles are lifted up when the ground freezes. When the ground thaws, the particles settle down again. Each time they settle down, they move a tiny bit farther down the slope because of gravity. Answer: gravity
Creep, which usually takes place where the ground freezes and thaws frequently, involves movement of particles due to what force?