Learn French: Ceux-ci sont écologiques. - These ones are eco-friendly.

The suffix -ci comes from ici (meaning "here"). It is attached to show that the objects are physically close to the speaker, translating to "these ones". If the objects were further away ("those ones"), you would attach -là to get ceux-là.

What does ceux tell us about the noun it's replacing?

ceux is the masculine plural demonstrative pronoun. This tells us the sentence is referring to a masculine plural noun that was just mentioned in the conversation, such as les sacs (the bags) or les véhicules (the vehicles).

How would this sentence change if we were talking about feminine objects, like houses (les maisons)?

You would need the feminine plural pronoun celles, making the sentence: Celles-ci sont écologiques. The adjective écologiques stays exactly the same because the singular form (écologique) already ends in an -e.

Why does écologiques end in an -s?

In French, adjectives must agree in gender and number with the noun or pronoun they describe. Since ceux-ci is plural, we have to add an -s to the singular adjective écologique.