Here’s more from the wonderful world of homophones (words that sound the same, but are spelled differently).
NOTE: Homonyms have the same spelling and pronunciation, but different meanings or origins, for example:
can¹ (modal verb) be able to – He can run a four-minute mile.
can² (modal verb) be allowed to – He can have a shotgun as he has a permit.
can³  (noun) a container usually made of metal – e.g. beer, paint and food cans

Anyway, back to homophones. Let’s take a look at rein, reign and rain, and they are all pronounced the same as the one we know so well, the rain falling from the sky. So:

Horses are controlled by reins, the leather straps leading from the bridal to the rider’s hands. The horse can be reined in (slowed or stopped) or steered with these.  We also have reins for small children, to stop them running into traffic etc.

The reign (period of ruling power) of Queen Elizabeth II has lasted since 1952. Industry reigned supreme until the rise of IT. NOTE: When reign is used for a person other than a monarch (king or queen) it often sounds rather negative (e.g. the reign of Hitler or some long-ruling politician).

“The rain in Spain falls mainly on the plain” is a sentence used to practice ‘correct’ (i.e. southern educated British)  pronunciation. It rained so much this winter that there was severe flooding in many parts of the UK.
“Save it for a rainy day,” means: “Don’t use it or spend it now, save it for when we really need it.”