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tokens: have the same peak value as the sample shown in the same panel. Based on the periodicity of these brightenings, we inquire whether or not they are standing mode MHD waves. They could be standing sausage waves because they can produce spectral profiles with enhancements at both red and blue wings (see, e.g., Tian et al. 2016b ; Shi et al. 2019a , 2019b ). For a loop with a cross section less than 1′′, the period of the sausage mode should be less than 10 s (Nakariakov et al. 2003 ; Nakariakov Verwichte 2005 ), thus the period of the observed phenomena is too large to be a sausage wave. One possibility is a combination of sausage and kink modes, which can occur concurrently in the solar atmosphere (Morton et al. 2012 ). The sausage mode can produce the observed spectral profiles, while the kink mode allows the bright loops to move transversely and thus can provide the observed periodicity. The periodicity of the sausage mode cannot be resolved in the current observations due to the temporal resolution. This leads to the question of whether a flux tube consisting of sausage motions with speeds more than 50 km s −1 is stable. This should be investigated in the future numerical studies. Another explanation is that shocks can be frequently seen in sunspots with periods of about 3 minutes (see Tian et al. 2014 ; Hou et al. 2018 , and references therein). Because shocks could be the result of leakage of p-modes, they can drive quasi-periodic brightenings in the transition region (Yang et al. 2015 ; Hou et al. 2016 , 2018 ). While shocks are propagating along loops, they might drive transverse motions (i.e., kink motions) and lead to repetitive appearances and disappearances of bright loops in the field of view of the spectrograph slit. The observed quasi-periodic brightenings can be the responses of shocks interacting with fine structures in the loop apex. Further theoretical study is required to understand whether and how shocks can produce such phenomena. Another scenario is that they are produced by repetitive magnetic reconnection. Such reconnection can result from magnetic braidings due to the plasma motions of the footpoints of the loops in the photosphere (Parker 1983a , 1983b ). The multiple loop threads seen in the field of view satisfy the basic requirement of this magnetic braiding scenario. Such magnetic reconnection can produce the observed non-Gaussian profiles of Si iv 1394 Å (see e.g., Huang et al. 2017 , 2018b ). Because the driven processes of the magnetic braiding can be modulated by the omnipresent 3 minute oscillations in sunspots, the braiding-induced magnetic reconnection then occurs with a periodicity. It has been demonstrated by numerical simulations that magnetic reconnection modulated by p-mode waves can produce quasi-periodic transition-region explosive events (Chen Priest 2006 ; Heggland et al. 2009 ). Moreover, the periods of the brightenings also agree with standing torsional (Alfvén) modes in a flux tube with lengths