As online education has exploded in the past decade, the number of physics and physics-related massive open online courses (or MOOCs, to use the industry lingo) has grown substantially as well. Several such courses aim at high school students, especially those involved in the AP Physics program. One of us (BK) was involved in creating two such courses, both of which use the edX platform: MechCx, a full AP Physics course, as part of the group led by David Pritchard at MIT and a smaller-scale MechC101x, where BK was the sole author and developer. CL, a senior at Weston High School in 2016-17, took MechC101x during its first run in the summer of 2015 before taking BK's AP Physics C class in 2015-16. The purpose of this article is to present the data obtained from the maiden run of MechC101x, analyze the advantages and disadvantages of MOOCs compared to face-to-face courses, and to lay out some principles for the effective use of MOOCs in physics education.