Thick means stupid in the UK, in case it isn't a commonly used term in the US.
So today I had to go to a separate building (under the same company I work for) because the guy who usually cleans there is off this week. I was given a card with the door code on, and I tapped it in. But I couldn't get the door open no matter how many times I tried. I knocked loudly on the door to see if there was anyone inside who could open the door, but nobody did.
So I called my boss and told him that I couldn't get the door open, and he had to come and sort this out. Wait for it...turns out I was turning the knob the wrong way.
I felt so stupid. Normally I'm a good problem-solver, so you'd think that I would have thought to try turning the knob the other way, but it just did not occur to me to do that at all. So the boss had to stop what he was doing and come all the way there for nothing. I just felt so embarrassed. Nobody's that stupid. An 80-year-old with dementia would probably have worked it out better than me.
I'm so stupid. Where's my brain?
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Female