Y can replace à + thing / object / location (adverbial pronoun)

You've already learned that the pronoun y is used to mean there (See Y = There (adverbial pronoun)).
Here is another usage of y. 

Look at these sentences:

Nous pensons à faire nos devoirs. Nous y pensons.We're thinking about doing our homework. We're thinking about it.

Il pense à ses vacances. Il y pense.He's thinking of his holidays.  He's thinking of it.
(Note that y can replace plural things too)

Elle répond à une lettre. Elle y répond.She's answering a letter. She's answering it.

Nous nous intéressons au sport. Nous nous y intéressons. We're interested in sport. We're interested in it.


Note that the pronoun y can also replace a group introduced by the preposition à + [thing(s)]/[object(s)]/[location(s)] (but not a person), particularly with verbs that require à.  
In such cases, you cannot replace the noun alone with a pronoun, you must replace the whole à + [noun] group with y.

ATTENTION:
In the case of people, the group à + [people] will be replaced by an indirect pronoun. See Replacing people with lui, leur = him, her, them (indirect object pronouns) and Me, te, nous, vous = Me, you, us, you (direct and indirect object pronouns).
 

Also see  En can replace de + phrase (adverbial pronoun) 

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Examples and resources

penser à


Nous pensons à faire nos devoirs. Nous y pensons.We're thinking about doing our homework. We're thinking about it.
Il pense à ses vacances. Il y pense.He's thinking of his holidays.  He's thinking of it.

répondre à


Elle répond à une lettre. Elle y répond.She's answering a letter. She's answering it.

s'intéresser à


Nous nous intéressons au sport. Nous nous y intéressons. We're interested in sport. We're interested in it.
I'll be right with you...