Why is dolphin not a fish?

We have been sorting animals into groups – dolphins have lungs and whales; so they are mammals not fish (which have gills).
We went outside and measured the length of a Blue Whale, which is 30 metres long.
In English, we have been learning about an information text, specially a persuasive text. We have focused our learning on asking questions, which need a capital letter and question mark.
For Maths we have been telling time – our focus has been quarter past, o’clock, half past and quarter to the hour. Today, we learnt about the days of the week.
Thought of the week: “You can fill the world with sunshine.” Snow White
Word of the week: question