Large language models have been shown useful in multiple domains including conversational agents, education, and explainable AI.ChatGPT is a large language model developed by OpenAI as a conversational agent. Being a large language model, ChatGPT is trained on massive amounts of data. Clearly, the characteristics of the data influence ChatGPT's responses. In this paper, we stress a surprising bias of ChatGPT related to the use of digits in numbers. Namely, we show a very high correlation between the frequency of digits generated by ChatGPT and humans' favorite numbers, with the most frequent digit generated by ChatGPT, matching humans' most favorable number, $7$. We also show some advantages of ChatGPT being developed as a conversational agent, and discuss some of its limitations.