Why does the Moon seem to change shape in the sky?

Answer: We see different lit portions

We see different lit portions ✓ — Correct! The Moon doesn't change shape - it's always a sphere. As the Moon orbits Earth (taking 29.5 days), we see different portions of its sunlit side. When the Moon is between Earth and Sun, we see mostly the dark side (new moon). When Earth is between them, we see the full sunlit side (full moon). The changing shapes are called phases.

The Moon spins irregularly — Wrong. The Moon rotates steadily, taking the same time to rotate as to orbit Earth (27.3 days). Irregular rotation wouldn't create the predictable monthly pattern of phases we observe.

Clouds block parts of it — Wrong. Clouds can temporarily block our view of the Moon, but they don't cause the regular monthly cycle of phases. The Moon's phases occur even on perfectly clear nights and follow a precise 29.5-day schedule.