There's this co-worker who I think has been flirting with me. The other day he asked me if I wanted to have a drink with him after work. But he sounded so casual that I didn't even think he was being serious and just said I was too tired. And that was also before I realised he was flirting with me. Today he asked me what I was doing tonight and I said "I don't know... I'm probably just going to sleep". He's 4 years younger than me and I know sometimes I come off a bit too strong. Could it be that he's was trying to ask me out again? What should I do to show him that I'm open to go out with him?