I'll bet
When someone says something you know is a complete lie, you say I'll bet like you're daring them to fail.
You say you never miss a day of school. I say I'll bet. You missed three days this week.
He says he never eats pizza. I say I'll bet. He's eating it right now.
She says she never gets distracted. I say I'll bet. She's texting her friend right now.