You can certainly try

"You can certainly try" is a signature phrase Matthew Mercer often uses when players ask if their characters can do something in-game. Other members of the cast have adopted the same phrase.

Players react to the phrase with apprehension, as the phrase reminds them of the uncertainty of the outcome and the potential consequences of failure; Taliesin Jaffe has said that "You can certainly try" is the darkest thing they ever hear in-game. The tone of the phrase can signal that an attempt is going to be especially difficult. But as Mercer explains, "It's D&D: If you can imagine it, you can certainly try."

Philosophy
Matthew Mercer has on occasion discussed his use of the phrase. In a video offering advice for dungeon masters in 2016, he brought it up in the context of the "Rule of Cool":

Notable uses
The first time the phrase was used onstream was in, when Laura Bailey asked if Vex'ahlia could talk to Nostoc Greyspine to distract him from Vax'ildan's attempt to steal beer, when Nostoc had already told the party it was time for them to go.

Sometimes, the use of "You can certainly try" is a clear signal of extreme difficulty. Liam O'Brien asked an exploratory question about the difficulty of pickpocketing a fire giant, to which Mercer replied that it would involve a vertical leap of eight feet: "You're not pickpocketing a giant without some high rolls. Mind you, you can certainly try. I would love for you to try."

After Travis Willingham rolled a natural 20 for Fjord to persuade Kima of Vord to let Yasha Nydoorin borrow the precious Holy Avenger, Mercer remarked that it was a low-probability "'You can certainly try' moment" that incredibly worked.

The first time one of the players used "You can certainly try" as a DM was in, when Liam responded to Sam Riegel saying he would try to stand up. Taliesin Jaffe later got to use the phrase in, to his delight.

When Jon Heder's character Lionel Gayheart wanted to pickpocket a bullet from Percival de Rolo, Taliesin told him, "You can try. You can certainly try." Lionel succeeded.

In-character
Occasionally, characters in-game will say they can "certainly try". Examples from the first campaign include:
 * A blacksmith responding to Tiberius Stormwind's request to add blades to the end of his quarterstaff.
 * Another blacksmith responding to Tiberius's request to forge several daggers into a Krull glaive.
 * Zanror weighing Vex'ahlia's idea to offer the bodies of supposed traitors as tribute to entice the ancient black dragon Umbrasyl into a trap.
 * Allura discussing the difficulty of trying to use her magic to locate Hotis in the Nine Hells.
 * When Percival asked Pike Trickfoot whether she could bless his silver ammunition to make it even more effective against the kind of evil creatures they might encounter in Thar Amphala, she replied, "I could certainly try."
 * Sprigg asked Pike if she could help him reach Ioun through prayer, to which she again replied, "I could certainly try."

In-character uses of the phrase in the second campaign include:
 * Given the idea of using Magician's Judge to penetrate the magical defenses of the Gear Warden, Yasha Nydoorin said she would certainly try.
 * When asked whether she could make a convincing illusion to help draw out the defenders of the Sour Nest, Nott said, "I can try—I can certainly try!" The episode was recorded live, and the audience cheered at the use of the phrase.
 * When Fjord wondered whether mithral trinkets could be melted down into one ingot more suitable for being transformed into iceflex, Umagorn Smeltborne said, "I can certainly try."
 * Jester Lavorre asked Allura if she could scry on the Laughing Hand to confirm that the destruction of the Permaheart had the desired effect. Allura responded, "I can certainly try. It's not easy."
 * In response to Jester's idea of appearing like Vence Nuthaleus to get information from Vence's alleged co-conspirator Adeen Tasithar, Essek Thelyss responded, "You can certainly try." Unbeknownst to Jester, Essek had framed Adeen as a traitor.
 * Veth Brenatto asked whether Beauregard Lionett could guide the party down through Cognouza to the Aether Crux. Marisha Ray looked to Mercer for guidance; he said that Beau could certainly try, and Beau said, "I can certainly try."

In the third campaign, in-character uses of the phrase include:
 * In response to the idea of combining their small groups into one party to get what each group wants, Orym says, "Certainly try it. Once."

Examples of the phrase being used by characters in one-shots include:
 * In, Tarvis ruminating on using Ray of Frost to freeze a trap's pressure plate in place.
 * In, when asked about the feasibility of one solution to a growing problem onboard, Ashley Johnson's AI character "Mother" said, "You could certainly try."

Trivia

 * Mercer has said he would eventually like to create a shirt with the phrase "You Can Certainly Try."