Grandir in the future anterior

Question

Regarding 'Ils ________ pendant les vacances." you give the answer as 'auront grandi'. Reverso gives both an avoir and an etre form forr the future anterior of grandir. I assumed as this was intransiitive it would be the etre form?

Accepted Answer

Bonjour Chris, 
Look at the following examples:

- Ils auront grandi pendant les vacances = they will have grown during the holidays
→ intransitive verb with auxiliary 'avoir'
-> when "grandir" = to grow (in size/height) -> use 'avoir' as auxiliary

- Elles se seront grandies avec des chaussures à talons = they will have made themselves taller with high heel shoes
→ intransitive verb with auxiliary 'être' / doing the action to oneself - not naturally

- Cette victoire t'a grandi = this victory has made you look worthy
→ transitive verb ('te' = direct object) with auxiliary 'avoir'

I hope this is helpful. 
Bonne journée !

Answer

Thanks Celine, I understand the examples but still not why Ils auront grandi pendant les vacances has the auxiliary in avoir, rather than the usual etre for intransitive forms of some verbs.

Answer

Bonjour Chris,I have edited my answer above. I hope this is helpful.Bonne journée !