You can use what is called as Spike. This is not a Scrum specific word. If your product owners asks you to build a robot and you are a software developer who has never done that , then it is obvious that you will not be comfortable giving a sizing or estimate on that story. Instead you can write a story called a Spike. This is a real story that your team will work on .

The intent of the story is to understand the actual story better Example: Investigate a build vs buy option Write some code that gives you enough confidence to understand the complexity involved in building a robot. Compare options example you may write some code to compare different OR mapping layers like Stored procedure, Hibernate , Object databases etc In the end estimation is always a guess. You can never be perfect You will get better at it as your team works together on that domain for a a while.