rèn zhēn
人家说着玩儿，你怎么就认起真来了？
Rénjiā shuōzhe wánr, nǐ zěnme jiù rèn qǐ zhēn lái le?
They were just joking around, why did you take it seriously?
认真学习
rènzhēn xuéxí
study seriously
工作认真
gōng zuò rèn zhēn
Work conscientiously / Work seriously.