One crab in a bucket without a lid will climb up the side of the bucket and escape. If there are two or more crabs in the bucket, there is no need for a lid.
You see, a single crab will see the opportunity of escape above and will climb upwards and succeed in reaching the top. You need to put a lid or cover on the top of the bucket to keep the single crab from getting out. If you have two or more crabs in the bucket, you need no lid. None of the crabs will be able to escape.