A: Hey, i got news from the principal that i won the math competition!
B: are you sure about that?. i thought the winner is Andy
A: yeah, i'm pretty sure. because i got an email too from the competition manager. here, you can see.
B: oh yeah, you're right. Congratulations!
Asking certainty :
  are you tired ?, i worry about your feet get hurt .
giving certainty :
  no i'm fine , yes i'm tired