"....they were all gazing, open-mouthed, appalled, at what was happening to the man's head. It was shrinking very fast, growing balder and balder, the black hair and stubble retracting into his skull; his cheeks becoming smooth, his skull round and covered with a peachlike fuzz… A baby's head now sat grotesquely on top of the thick, muscled neck of the Death Eater as he struggled to get up again; but even as they watched, their mouths open, the head began to swell to its previous proportions again; thick black hair was sprouting from the pate and chin…" (OP35)
The first time Harry, Hermione, Ron, Neville, Ginny and Luna, who have broken into the Department of Mysteries, discover the bell jar inside the Time Room, there is a tiny hummingbird inside its "glittering wind". The bird transforms itself through its lifecycle: from being an egg at the bottom of the jar to a fully grown bird as it floats to the top. It becomes an egg again as it drifts back to the bottom (OP34).
During the battle with Bellatrix Lestrange and Voldemort's Death Eaters, the head of one of the Death Eaters falls into the bell jar. His head then starts to quickly move through time - becoming a baby's head and then back to a man's, finally settling as a baby's head when he removes the jar (OP35).
A "bell jar" is a straight sided glass cylinder with a rounded dome top. They are used for scientific experiments to form a vacuum and were also popular since Victorian times as decorative covers for plants, clocks and cheese.