Twinkling or Sparkly Houses

[dimnd97]

I'm new at City Story Metro (level 5) and, when I logged on this morning and collected from my buildings, some of my houses started twinkling. I thought maybe community members had visited, but I couldn't collect anything more and found no messages indicating they had. I can't find in the instructions or on this forum why they are doing that. What is the purpose?

[rwhiggs]

I have the same issue; came here for an answer

[totaltechno]

Next time you collect from that building, you'll get 20% more.