Large hourglasses with different symbols inside or on top of them.
Hourglasses are objects in the Sonic the Hedgehog series, which basically function similar to regular switches. When "activated" (flipped over by an attack), it is usually set for an amount of time before the switch "deactivates". The Hourglasses are usually decorated with either the bumper symbol on top or bottom plates of the hourglass.
Large hourglasses are first seen in Sonic Adventure 2, where it can be found at Dr. Eggman's pyramid base's stages, such as Pyramid Cave, Egg Quarters and Death Chamber. If the player jumps on or punches an hourglass, it will flip and various doors will open for a certain amount of time. However, the doors slowly close as the sand in the hourglass runs out. The player is able to determine how much time is left by listening to a sound much like a clock ticking in the background, which steadily speeds up as the sand falls. The player can then travel back and activate the hourglass again in order to proceed on the stage.
When it comes to the fight against King Boom Boo, one of his minions conveniently carries an hourglass. Hitting the minion will flip the hourglass, opening doors in the ceiling and letting the sunlight in, which will get King Boom Boo in his weak shadow state.
The hourglasses in Sonic Unleashed function differently than in their previous appearance. Other than the option to change Day or Night in the pause menu when at Town Stages, there is an hourglass in each Entrance Stage which has the same function of switching from day or night and from hedgehog to werehog. Hourglasses are also set on numerous sections in Eggmanland, where the player has to transform between hedgehog and werehog most of the time in order to progress in the large stage.
Apperances in other media
In the episode "Sonic's Scream Test" of the anime series Sonic X, there was a stone figure of an hourglass called the "talisman" similar to the ones in Sonic Adventure 2. It had a figure of a sun on one end and a crescent with a star (with the star being removed from the English dub) on the other end. It was discovered on the grounds of a castle where a movie starring Lindsey Thorndyke was being shot. One of the cameramen removed the talisman statue, which awoke King Boom Boo and the Boos. Restoring the talisman to its rightful place made the Boos depart.
In the Archie Comics' Sonic X #14, the talisman reappears in attempt to use it against the reawakened King Boom Boo and his Boo minions by Cream. As a promise to not disturb the heroes, King Boom Boo orders that the talisman to be placed back at its pedestal by dawn of the following day.