Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: With the naked eye, bacteria just look like a slimy smear on a petri dish. How can we study them in more detail? The invention of the microscope has allowed us to see bacteria, cells, and other things too small to be seen with the naked eye. Answer: slimy smear

Answer: How does the bacteria look when seen with the naked eye?

Question: Passage: Graphs are very useful tools in science because they display data visually. Three commonly used types of graphs are bar graphs, circle graphs, and line graphs. Each type of graph is suitable for a different type of data. Answer: graphs

Answer: Bar, circle, and line are examples of what kind of data visualization?

Question: Passage: External forces are forces that act on the system from outside. In our previous example, external forces include the force of gravity acting on both cars (because the other part of the force pair, the pull of gravity the Earth experiences coming from the cars, is not included in the system) and the forces of friction between the tires and the road. Answer: external

Answer:
What do we call forces that act on the system from outside?