What is the difference between animal and insects?

Yesterday while I was watching the television I came across this commercial...

WHAT IS THE MOST DANGEROUS ANIMAL?

and the answer to that question posted on television just after the question is...

THE FLY

and my kid (8 years old) immediately asked me, is the fly an animal or an insects? It took me quite some time to think of the most accurate answer to the little child question.

How exactly do we differentiate between an insect and an animal? Shouldn't it be an animal is a lot more bigger than an insect? Hence a fly should fall under the category of an insect and not an animal? This situation creates a confusion among kids especially when a famous TV station displays such an advertisement which is meant to expose children for general information.

I would humbly advise them to use the most suitable words for their info advertisement!