Waiting in line to visit a clock is a paradox of wasted time folding in upon itself. This skip-the-line ticket gets you right in, so you can visit the Gothic clock tower and see the insides of the world's oldest operational astronomical clock.
Meet your assistant at the Get Prague Guide office to pick up your tickets. While picking up the tickets you'll have a 20-minute introduction about the history and main information on the tower, so once you get there it will feel like you know the place.
Then, visit the clock on your own. Every hour two small trapdoors open and the twelve apostles march out to tell their stories. The Grim Reaper also rings a bell, checking to see whose time has come (unsurprisingly, everyone shakes their heads 'no').
There are also planetary motion, the zodiac and more on the clock - and 75% of its parts are original, like 'from-the-15th-century' original.
You'll get to skip the lines and take the stairs or an elevator up the tower. From there you'll have amazing views over the Old Town Square and the rest of Prague's Old Town.
On your way back down, you can think on the staying power of this clock. It's an appropriate occasion to call to mind the words of English essayist Henry Austin Dobson:
Time goes, you say?
Ah no! Alas, Time stays, we go.
That ought to encourage you to make the most of your time in Prague.