Plastic either gets harden (and then crack), or soften (and then break) over time. There is no exception to the "mouse button" of my Sony Vaio U3.
To fix it, I have replaced it with a soft pad which is of similar size, the motion is now perfect.
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I always wanted to expand the capacity of my Sony U3, and a memory stick seems a good option. However, after some research, I found that the memory stick slot supports only the original plain old memory stick up to 128M only, so it makes no sense to buy a 4G one for it.