Fresh cut grass reminds me of playing outside as a kid while my dad cut the grass on Sundays.
Chocolate mixed with a hint of natural vanilla reminds me of the cookies that my grandmother used to bake when my parents were gone and she was babysitting.
Burnt hair reminds me of that one time in high school chemistry class.
The way that scent can so quickly trigger a memory is actually a well-documented phenomenon, called the Proust Phenomenon. Marcel Proust first wrote about this in Remembrance of Things Past, in which he describes how the scent of a Madeleine cookie dipped into a cup of tea inevitably reminds him of moments in his childhood when his mother and aunt served a similar snack. The slightly more scientific name for it is an "odor-evoked autobiographical memory."
But, why does this happen?
Scientific studies show that scent enters your body via the olfactory bulb, and then enters your brain very quickly--it takes only two nerve synapses for scent information to enter your brain, as compared to up to sixteen synapses for visual information to enter. And, unlike other sensory information, this scent data begins in the amygdala and hippocampus, two regions of the brain which primarily deal with memory. Therefore, this link between your hippocampus and your olfactory ('smell') system is the likely explanation for scent triggering old memories!
What scents trigger certain memories for you? Feel free to share in the comments!