Is the sentence correct?
Yes — the sentence is grammatically correct.
Correct sentence: The fox surely has the puffiest tail of all.
What the words mean (for a 7‑year‑old):
- fox — a small, clever animal with a bushy tail.
- surely — means you are very sure about something.
- puffiest — the most puffy; the fluffiest tail compared to others.
- of all — means compared to everyone or everything else.
Why it is correct: The sentence has a subject (the fox), a word that shows certainty (surely), a verb (has), and a superlative describing the tail (puffiest) plus the phrase (of all). The word order and punctuation are fine.
Other correct ways to say it:
- The fox has the puffiest tail of all.
- That fox surely has the puffiest tail of all the animals.
If you want the sentence to sound simpler, you can remove "surely": The fox has the puffiest tail of all.