Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only contract.
Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?

Ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean are called coral reefs.
Ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean are called what?

El niño is the cause of rising surface temperatures of the pacific ocean, drought in the western united states, and flooding on the pacific coast of south america.

What is the cause of rising surface temperatures of the Pacific Ocean, drought in the western United States, and flooding on the Pacific coast of South America?