Complex sentence: The Ice Bucket Challenge is an activity involving dumping a bucket of ice water on one's head, which was started as a campaign to promote awareness of amyotrophic lateral sclerosis (ALS), and gained widespread popularity in 2014.
Split into simpler sentences: The Ice Bucket Challenge is an activity involving dumping a bucket of ice water on one's head. It was started as a campaign to promote awareness of amyotrophic lateral sclerosis (ALS), and gained widespread popularity in 2014.