Although they could not be built with the materials of the time he invented flying machines. Da Vinci approached science and art in the same manner that he made sketches to help him solve problems. He saw no difference between planning a machine and planning a painting. Da Vinci is probably best known for his painting The Mona Lisa . The following paragraph has shifts in verb tense.
Scientists perform a lot of experiments to observe important things.
For example, scientists raise fleas in labs to learn more about how these pests caused disease in humans. The fleas are kept in special jars that contain sieves. The fleas lay eggs in the jars, and these eggs drop through the sieves. The scientists collected the eggs so they can raise more fleas from them . The jars also have tubes that carry warm water to heat blood that is contained underneath a skin-like sheath.