How to Tell Your Boss That You Lost Your Key

Last week after working at my current job for nearly two and a half years, my boss finally granted me lock up privileges. When he gave me the key, he said, no, don't you lose this. And throughout the week, he kept telling me to take good care of it. At the end of the last day, before I was going to open the store and run myself with the first time, he said,. You haven't lost a key? I got so annoyed that I kind of sat, no, I haven't lost it since the last time you asked. So please, how the hell do I tell my boss that I lost the freaking key?