Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: All that magma and gas pushing upward can make the volcano's slope begin to swell. Ground swelling may change the shape of a volcano or cause rock falls and landslides. Most of the time, the ground tilting is not visible. Scientists detect it by using tiltmeters, which are instruments that measure the angle of the slope of a volcano. Answer: tilt meters
Output:
Instruments that measure the angle of the slope of a volcano are called what?