happen upon (someone or something)
to meet someone or find something unexpectedly
I happened upon a very valuable book when I was cleaning up my grandfather's house.
Jumping Jacks in a Flash
The real beauty of jumping jacks is that, for all the health benefits, you dont need to invest money in equipment or devote a great deal of time to reap the rewards. You know the drill: Spread the arms and legs as you jump, then jump again, bringing the arms to the sides and the legs together. Brown recommends 60 seconds of jumping jacks to begin, gradually increasing by 10 second increments as your endurance improves.