you know your stupid when

Ok I take my half blind wife cindy to Kingston Ontario today about 50 miles away to an appointment for an eye check up.On the way there in our old 99 sunfire when the temp goes a tab past what i like to see no over heating yet.So we get there ok,after 2 hours I check the coolant well maybe add a little water,now here is were i am STUPID,I forgot to put the cap back on the plastic tank drove home no problims untill I pull off the highway than starts to steem up and heat up to the point it reads in the red.Got it home let it cool a bit went around the block dam back to reading in the red again.Ok off to a shop I trust it had it toad there, cools down service manager say its fine.I am waiting for my buddy to call tonight to tell me what he did or not did.I dont understand why it would heat up and now be fine.Sorry this is long but I feel very STUPID right now.#-o