Bracket Inserter
Mark off the extra information in this sentence with a pair of brackets. Brackets are the pair that whispers: what is inside is an extra the sentence can live without.
A parenthesis is an aside: drop it and the sentence is unharmed. "Scientists measured the whale at thirty metres." proves it. The pair of marks goes tight around the whole aside and nowhere else - one mark before "longer" and one after "field" - giving "Scientists measured the whale at thirty metres (longer than our school field)." Because the aside runs to the end, the closing bracket sits INSIDE the full stop: the full stop belongs to the sentence, not to the aside. A dash or a comma could not close it here at all, because there is nothing left to pair with.
What to say if they get stuck
- Lift a run of words out. If the sentence still stands up on its own, that run is the extra bit.
- The sentence works perfectly well as "Scientists measured the whale at thirty metres." - so everything you took out is the aside.
- Open before "longer" and close after "field" - and the closing bracket goes INSIDE the full stop, not after it.