Today is the legendary Broadway composer Stephen Sondheim's 89th birthday, and if there's one thing more difficult than enduring 89 years on this planet, it's singing one of his songs.

Regarded by many as the greatest musical theatre composer of our time, Sondheim's songs are famously intricate, challenging singers and accompanists alike. Whether it's a ballad with a heart-wrenching emotional journey, or a fast-paced, lyrically challenging bop with a minefield of consonants, the same qualities that make Sondheim legendary are widely the same qualities that make his work intimidating to actors everywhere.

Here are some of the trickiest—and most rewarding—Sondheim numbers.

"Your Fault" from Into the Woods

Performing "Your Fault" from into the woods is like being a trapeze artist: the stakes are high and if you want it to work, you need the other person to catch you. The actors in this song have to be completely focused to take over the lyrics at the precise time, and with lyrics like "It's his father's fault that the curse got placed and the place got cursed in the first place," you don't want to miss a single word.

"Send in the Clowns" from A Little Night Music

At face value, this famous ballad may seem easy—but that's precisely why the song shouldn't be performed at face value. Though it's slower than most of Sondheim's famously difficult works, Dame Judi Dench proves here that cultivating the inner life that disappoints the character Desirée so much is no easy feat, but the palpable heartbreak it results in is the reward.

"Simple" from Anyone Can Whistle

The "Simple" sequence from Anyone Can Whistle is, in fact, anything but. With a bevy of characters and unpredictable beats, this 13 minute sequence is a roller coaster ride of Sondheim at his Sondheim-est.

"Worst Pies in London" from Sweeney Todd

Business may be slow at Mrs. Lovett's pie shop, but this song will have you asking, "What's your rush? What's your hurry?". It's easy to get breathless just listening to it. Not to mention Mrs. Lovett's baking process tends to be pretty aerobic, as the brilliant Patti LuPone demonstrates here.

"Not Getting Married Today" from Company

"Do you want to see a crazy lady fall apart in front of you?" Get them to sing this song. A sane composer would have said "There's no way a human being could perform this," and that's why Sondheim is a genius. Even though he puts actors through the wringer with his words, the challenge so often results in their best work.