In the novel, that hourglass was her power awaken given form when she got killed, wishing "to turn the hourglass". To all other, it just a normal hourglass, only to Aria, it act as her magic trigger. And it got explain much later that magic power from body is consume to perform magic, which is why she immediately got tired for next whole day. Even this testing/mischief phase show the weakness of the hourglass very soon so Aria can become aware of it, even led to prepare for failure situation