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stiffen [stiffened, stiffening, stiffens]▼verb[UK: ˈstɪf.n̩] [US: ˈstɪf.n̩]
strengthen [strengthened, strengthening, strengthens]▼verb[UK: ˈstreŋ.θn̩] [US: ˈstreŋ.θn̩]
toughen [toughened, toughening, toughens]▼verb[UK: ˈtʌf.n̩] [US: ˈtʌf.n̩]
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