Testers do some of their most important work during story writing and release planning.
During story writing and release planning, a tester should
Identify and make explicit hidden assumptions
Define some acceptance tests for each story
Estimate acceptance-test time and include it in story estimates
Enable accurate estimates of time and velocity
Accomplishing these lay the groundwork for
More accurate implementations
Time for automation
Testing that "keeps up"
Happier customers
Here's a warmup exercise.