Hello! Let's look at the sentence: 'A blue whale’s mouth can fit a car.'
The sentence is almost correct, but to make it clearer and grammatically right, we can say: 'A blue whale’s mouth can fit a car inside.'
Or you can say: 'A blue whale’s mouth is big enough to fit a car.'
This means the mouth of a blue whale is very big – so big, that a car can fit inside it!