Can I play offline?

Yes. Once you have opened the game, it keeps working with no signal. Your answers are saved on the device and sent when you are back online. The Daily Sprint, Make a Study Game and the live leaderboard need a connection.

Yes. After you have opened /play/ once, the game keeps working without a connection.

Cards, sounds and jokes play from your device's copy.
Your answers are saved on the device and sent to your account when you are back online. Your score then catches up.
Your scores screen shows this device's numbers while offline.

These need a connection:

the Daily Sprint and challenges,
Make a Study Game and the study game library,
the live leaderboard, signing in and certificates.

Tip: install SESK on your phone first (see "How do I install SESK on my phone?"), then open it once before a trip.

New cards show up the next time you are online.