Many of us learn howto ride a bicycle at a young age. As we grow older, we may stop riding-most ofus go to school by bus or by subway. But whenever we get on a bicycle, ifs likewe've never stopped biking.

This is strange. Inmany other cases, our memories let us down. We often find it hard to rememberthe name of a person or a place, don't we? So, why can we still ride a bicycle afterwe stopped riding a long time ago?

It turns out thatthere are many types of memory. They are stored in different regions of ourbrains. Memories of experiences (e. g, our first day at school) and of factualknowledge (e. g. , the capital of France) are called declarative (陈述的)memories. These memories can be communicated to others. Skills suchas playing an instrument or riding a bike： are calledprocedural(程序的)memories. These memories are responsiblefor our performance.

Scientists have donemany experiments on different memory types. Perhaps the most famous of them wascarried out on Henry Gustav Molaison, an epileptic (癫痫患者). In an operation in the 1950s, part of his brain was taken away.After that, the man suffered less- from epilepsy. But there was a newproblem-he couldn't remember things!

What went wrong withMolaison's memory system? Through various tests; scientists found he couldlearn new skills (slowly, though). However, he could never remember having beentrained to learn a new skill. In other words, Molaison could develop newprocedural, but not declarative, memories.

Scientists haveconcluded that even with serious brain injuries, procedural memories last muchlonger. However, they haven't yet found out why. Some say procedural memoriesare more lasting because they are stored in the brain's center.

Now, "just likeriding a bicycle55 has been used as an idiom (习语).Can you guess its meaning? Yes, people use it to describe a skill that, oncelearned, is never forgotten.