  Currently, FRB 121102 is the only fast radio burst source that was observed
to give out bursts repeatedly. It shows a high repeating rate, with more than
one hundred bursts being spotted, but with no obvious periodicity in the
activities. Thanks to its repetition, the source was well localized with a
subarcsecond accuracy, leading to a redshift measurement of about 0.2. FRB
121102 is a unique source that can help us understand the enigmatic nature of
fast radio bursts. In this study, we analyze the characteristics of the waiting
times between bursts from FRB 121102. It is found that there is a clear bimodal
distribution for the waiting times. While most waiting times cluster at several
hundred seconds, a small portion of the waiting times are strikingly in the
range of 2--40 millisecond. More interestingly, it is found that the waiting
time does not correlate with the burst intensity, either for the preceding
burst or for the subsequent burst. It strongly indicates that the repeating
bursts should be generated by some external mechanisms, but not internal
mechanisms. As a result, the models involving collisions between small bodies
and neutron stars could be competitive mechanisms for this interesting source.
