They are normally dim and constant-- the most you'll see in a dark room is a faint glow of twinkling like the night sky. When you set your glass down on the table, however, the table "sees" the motion that induces a slow ripple starting in that location that spreads out to other areas of the table as it dissipates. The overall effect is a bit like touching a pool of water, in that your local disturbance turns into an overall gentle rippling, and eventually settles down.
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