It is a question constantly asked by two distinct sets of humans: 1) adolescent children curious about how the world works; 2) beat reporters.
The third-year pro was at a loss for an explanation, saying it wasn't his ankle injury nor his weight that has slowed him this season.
"My ankle's cool," he said, per the Milwaukee Journal Sentinel, and insisted any added weight is not influencing his poor play. "No," Lacy said, twice, when asked about his size being a problem.
"I honestly don't know what it is," Lacy said. "Definitely, I'm off to a different start. I'm not playing how I was the previous two years. Later on in the season, maybe it'll pick back up. But as of right now, this is just the move that is going to be.
"This season (Starks) has definitely been the better player between the both of us. That's just the move."
If it's not injury or weight, then Lacy's play is apparently inexplicable. His foot speed is categorically slower than last season and he strains to get to holes before they close.
Lacy is averaging 3.7 yards per carry on 83 attempts for just 308 yards. In his last four games, Lacy has a combined 78 yards rushing and hasn't earned more than 38 ground yards, while tallying more than 2.1 yards per rush only once.
Lacy admitted the demotion is a stumbling block, but believes he can get back on track.
"Who knows how long it's going to be like this," Lacy said. "Maybe things pick back up for me and it's right back and we're never having this discussion again.
"You don't know where this could lead to. But as of right now, for the team, this is pretty much the best move."