Scry

Scry is a keyword action that allows a player to look at a certain number of cards from the top of their library and put them on the bottom of the library or back on top in any order. It primarily features on instants and sorceries, but has also appeared on creatures, artifacts, lands (including a cycle first printed in Theros block), and enchantments. It is also part of the Vancouver mulligan.

History
The first cards with scry were designed by Aaron Forsythe and printed in Fifth Dawn. It was then expanded on several years later in Future Sight, and returned in Magic 2011 and Theros block. It became evergreen in Magic Origins. When the mechanic became evergreen some older cards like Soldevi Excavations and Opt were errata'd to have the keyword.

Originally scry was a keyword ability like flying or first strike. It has now been errata'd to be a "keyword action", like regenerate, attach, or counter. The original cycle of Fifth Dawn instants and sorceries with scry has been errata'd to have a period after "Scry N", indicating that it is no longer a keyword ability but an action.

According to Mark Rosewater, any color can scry 1, but scrying more is limited to certain colors. Blue is the primary color to scry. It also gets the largest scry numbers with red getting the lowest.

In 2015, the Battle for Zendikar prerelease brought scry into the mulligan rules (then called the "Vancouver" mulligan). In those mulligan rules, all players that kept less than 7 cards in their starting hand would scry 1.

Rulings

 * When you scry, you may put all the cards you look at back on top of your library, you may put all of those cards on the bottom of your library, or you may put some of those cards on top and the rest of them on the bottom.
 * You choose how to order those cards no matter where you put them.
 * You perform the actions stated on a card in sequence. Some spells with scry, e.g. all of them in the Fifth Dawn set, have scry as the last part of their text, so scrying is the last thing you do before the spell is put into its owner's graveyard. Other cards (e.g. Read the Bones) have scry followed by another effect, so scrying is done before following the other instructions.
 * Scry appears on some spells and abilities with one or more targets. If all of the spell or ability's targets are illegal when it tries to resolve, it will be countered and none of its effects will happen. You won't scry.

Reminder text
As of Theros block, the latest reminder text for Scry differs depending on the number.

If N is 1 the reminder text reads as follows: "Look at the top card of your library. You may put that card on the bottom of your library.".

Scrycast
Scrycast is a mechanic that appeared on a test card in the convention version of the Mystery Booster set, on Biting Remark. If a card with Scrycast is seen while scrying, its owner may reveal it and cast it for its Scrycast cost.