Evaluate the similarity between them and classify them into classes from 0-5 as follows:
0 : The two sentences are completely dissimilar.
1 : The two sentences are not equivalent, but are on the same topic.
2 : The two sentences are not equivalent, but share some details.
3 : The two sentences are roughly equivalent, but some important information differs/missing.
4 : The two sentences are mostly equivalent, but some unimportant details differ.
5 : The two sentences are completely equivalent, as they mean the same thing.

Sentence 1: The baby is laughing and crawling. Sentence 2: A baby is laughing and crawling on the floor.
4

Sentence 1: David @OmSigDAVID, I think Santorum's more likely than Gingrich. Sentence 2: @sozobe, I think Santorum's more likely than Gingrich.
5

Sentence 1: Someone is cracking an egg open. Sentence 2: A man is breaking an egg.

3