Sometimes a story is too large or overly complex.
Perhaps the implementation or a 3rd party tool or library is poorly understood.
The team can’t estimate the story.
Perhaps we’re unsure if we’ll be able to complete the story due to some potential blocker.
In these cases, we might want to build a functional or technical experiment to figure it out.
We might want to look into something for a day.
We might want to look up alternatives; including googling or experimenting with some other library or software package.
Consider alternative refactoring paths.
These are “spikes”.