How can sky be plural if there is one sky?

In English a singular noun ending in y normally ends in ies in plural form: sky/skies; mercy/mercies; jelly/jellies; fly/flies; jetty/jetties; butterfly/butterflies

Sky is not plural. Skies would be the proper term if we had more than one sky. But we don't, so I think this info will be useless.

Skies.

You operate sky online as you would book any flight. You must make a user profile first and then proceed to follow the steps to be able to book flights.

There is nothing sold in a bottle that goes by the name of Sky One. Skyy vodka costs between $25 and $30 per liter. Sky One is the name of a digital television broadcast network in the UK.