Ask them to quickly (in one minute or so) write down or draw everything that is on their mind right now (worries, hopes, distractions, etc.).
Depending on size and nature of the group, have them share: all the things they have listed, the two most worriesome things, the one most important thing, etc.
Sometimes, after talking about trying to set aside troubling things so that learning can take place, it can be useful to have participants crumple up the papers and toss them into a box to signify a willingness to let go for now and focus on the task at hand.
Other times, depending on the group, it might serve as a springboard for discussing all the things that impact us, how we do our work, or how we relate to others (building group cohesion through sharing the important things on our mind).
We need to validate that people have a lot on their mind, that similar things affect each person in varying degrees, and that those things are part of what they bring to the learning experience.
This can work very nicely when working with a group who may be unhappy about working together—like a "walk a mile in my shoes" experience.