The second line of the Schedule C instructions say "An activity qualifies as a business if your primary purpose for engaging in the activity is for income or profit, and you are engaged in the activity with continuity and regularity. For example, a sporadic activity or hobby does not qualify as a business"
Now - what is sporadic? Last year I did two shortish consultancy jobs (got 1099s) but the were neither continual nor regular. I shall probably not repeat them. So, to my mind, I do not have a business. Yet I see from other entries that folk who did one small job were told they had to do a schedule C. What gives?