Low-frequency dielectric measurements on plexiglass (PMMA) show that coolin
g and heating the sample at constant rate give a hysteretic dependence on t
emperature of the dielectric constant epsilon. A temporary stop of cooling
produces a downward relaxation of epsilon. Two mail features are observed:
i) when cooling is resumed epsilon goes back to the values obtained without
the cooling stop (i.e. the low-temperature state is independent of the coo
ling history); ii) upon reheating epsilon keeps the memory of the aging his
tory (Memory). The analogies and differences with Similar experiments done
in spin-glasses are discussed.