Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: In the late 1600s, Isaac Newton introduced his law of gravity, which identifies gravity as a force of attraction between all objects with mass in the universe. The law also states that the strength of gravity between two objects depends on their mass and distance apart. Newtons law of gravity was accepted for more than two centuries. It can predict the motion of most objects and was even used by NASA to land astronauts on the moon. Its still used for most practical purposes. However, Newtons law doesnt explain why gravity occurs. It only describes how gravity seems to affect objects. There are also some cases in which Newtons law doesnt even describe what happens. Q: Newton expressed his ideas about gravity as a law. A law in science is a description of what always occurs in nature. For example, according to Newtons law, objects on Earth always fall down, not up. What is needed to explain gravity? A: A theory is needed to explain gravity. In science, a theory is a broad explanation that is supported by a great deal of evidence. In the early 1900s, Albert Einstein came up with a theory of gravity that actually explains gravity rather than simply describing its effects. Einstein showed mathematically that gravity is not really a force that of attraction between all objects with mass, as Newton thought. Instead, Einstein showed that gravity is a result of the warping, or curving, of space and time, which made up the same space-time fabric. These ideas about space-time and gravity became known as Einsteins theory of general relativity. Einstein derived his theory using mathematics. However, you can get a good grasp of it with the help of a simple visual analogy. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Now imagine placing a lighter ball at the edge of the trampoline. What will happen? It will roll down toward the bowling ball. This apparent attraction to the bowling ball occurs because the trampoline curves downward, not because the two balls are actually attracted to one another by an invisible force called gravity. Einstein theorized that the sun and other very massive bodies affect space and time around them in a way that is similar to the effect of the bowling ball on the trampoline. The more massive a body is, the more it causes space-time to curve. This idea is represented by the Figure 1.1. According to Einstein, objects move toward one another because of the curves in space-time, not because they are pulling on each other with a force of attraction. Einsteins theory is supported by evidence and widely accepted today, although Newtons law is still used for many calculations.
Question: einsteins concept of gravity is similar to what happens when you place a bowling ball on the surface of a trampoline. in this analogy, if the bowling ball represents earth, then the surface of the trampoline represents
space-time.