[Answer] What do elephants, rhinos, hippos and sloths have in common?

Step 1 : Introduction to the question "What do elephants, rhinos, hippos and sloths have in common?"

...Elephants, rhinos, hippos and sloths are all mammals who give milk to their babies when they're first born. They all have backbones, are hairy and produce their own body heat. This means these mammals spend more time with their babies. Because it takes more time they also share another trait: they can't jump. There are actually three types of mammals: eutheria (placental mammals), marsupials (also called metatheria) and monotreness. Source: Reference.com

[Ans] in 1884, who led 21 elephants across the brooklyn bridge to prove it was safe?

Step 1 : Introduction to the question "in 1884, who led 21 elephants across the brooklyn bridge to prove it was safe?"

The bridge—originally referred to as the New York and Brooklyn Bridge and as the East River Bridge— was opened for use on May 24, 1883. Thousands of people attended the opening ceremony and many ships were present in the East Bay for the occasion.