Tell them you took some time to calm down and assess the situation. Describe how you tried to resolve the situation on your own by asking the employee who took credit for your work what happened. Then describe what you learned from that exchange and what you did with that knowledge.

"I brought it to the attention of my supervisors right away. I should've been promoted, not that person. They should be aware that the person who stole my idea lacks integrity and doesn't deserve to be rewarded for something they earn."

A leader will ask you tried conflict resolution with your coworker prior to bringing it to their attention. They have other priorities and would prefer you handle it on your own when possible. Maybe they forgot you mentioned the idea and genuinely believed it was their own. Perhaps they intended to give you mutual credit as a collaborator and, under the spotlight, forgot. Perhaps they were too embarrassed to admit it after the fact. When working in a team, everyone should work together, and always be given the benefit of the doubt. When a team leader has to drop what they're focused on to deal with inter-personnel disputes, this can hinder their progress and timelines, and also hurt the functionality of the team.

"I've experienced this firsthand. I was pouring over my findings and came up with this idea. Just as it occurred to me, a colleague asked what I was working on. I told them, and they got excited and started helping me quantify my data. Later on, our group had to give a presentation, and my colleague delivered it. When our team leader followed up with some thoughts, my colleague contradicted their speculation with my findings and presented my idea as an alternate solution. The next thing I knew, they promoted them to another department. When I confronted them, they apologized for how things unfolded and told me they gave me credit behind the scenes. When reviews were delivered they showed appreciation."

"I think I'd wait for a good time to discreetly give them a chance to explain why they did so. There are two sides to every story, and there is no way I can scientifically draw a conclusion from assumptions. So first I'd need to collect as much data as possible. Then I'd need to evaluate what winning in such a situation looks like. Whichever course of action serves the greater good would be the appropriate course of action. Luckily, I trust the intentions of those on my team, and this hasn't happened to me."