Take 大 — the person with arms spread wide — and give them a huge, emphasized head. What is above your head? The sky. 天 began as "the top of a person" and grew into the grandest word in the language: heaven, the sky, and later simply "day".
A piece of culture
The Mandate of Heaven Around 1046 BCE the Zhou overthrew the Shang and needed to explain why that was allowed. Their answer shaped three thousand years of history: 天命, the Mandate of Heaven. Heaven grants the right to rule — and withdraws it from rulers who become unjust. Floods, famines, and lost battles were read as heaven changing its mind. Every later dynasty rose claiming the mandate and fell accused of losing it: history's longest-running job review.
Use it in a real word
天下
tiānxià · tin1 haa6
"all under heaven" — the classical Chinese word for the whole world