Third-person singular simple present indicative form of react.(verb)
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Use "reacts" in a sentence
"Then, by bringing mindful awareness to how your brain reacts to feeling threatened, you can stimulate and therefore build up the neural substrates of a mind that has more calm, wisdom and sense of inner strength."
"So basing your design on how you, the computer scientist, thinks/acts/reacts is faulty."
"Next step is to watch how Sarah Palin reacts to a piece of legislation she'll never read to see what "bogey men" she can find/create this time and then watch the White House hang her from her own petard in the process for all to see."