This study investigates the potential of ChatGPT and Bing Chat, advanced conversational AIs, as "objects-to-think-with," resources that foster reflective and critical thinking, and concept comprehension in enhancing STEM education, using a constructionist theoretical framework. A single-case study methodology was used to analyse extensive interaction logs between students and both AI systems in simulated STEM learning experiences. The results highlight the ability of ChatGPT and Bing Chat to help learners develop reflective and critical thinking, creativity, problem-solving skills, and concept comprehension. However, integrating AIs with collaborative learning and other educational activities is crucial, as is addressing potential limitations like concerns about AI information accuracy and reliability of the AIs' information and diminished human interaction. The study concludes that ChatGPT and Bing Chat as objects-to-think-with offer promising avenues to revolutionise STEM education through a constructionist lens, fostering engagement in inclusive and accessible learning environments.