Or a long ride in a car can trigger memories of family vacations (my relatives lived five hundred miles away). Or just seeing a toddler taking his or her first steps instantly calls up warm memories of my own children doing the same, and suddenly I'm teary-eyed. Happens all the time, no matter the fact that I'm terribly happy that my sons are grown and living happy, independent and productive lives. I don't wish them to be small again, but I'm so happy that I still retain vivid memories of when they were.
Songs are instant paths to the past. Sometimes even songs that have nothing to do with the time frame I'm remembering. Most of the time that's a good thing. Sometimes it can leave me slightly sad for days I'll never see again and (more importantly) people I'll never see or talk to again.
But most of the time, those sensory wormholes to memory lane are wonderful. The scent of Ovaltine (there was a factory nearby) or bakery (I lived upstairs from a bakery for awhile when I was a child) bring back lovely memories. A campfire brings to mind my honeymoon when we wandered for weeks, staying at nice hotels but also camping in the western part of the United States.
What types of things are instant memory reminders for you?