In case you thought the idea of cute cubs at the Toronto Zoo had grown tired ... one of them yawns.

Zoo staff today released a video of the adorably tired twin cub yawning.

The pandas are now 12-weeks-old and weigh more than 3.5 kilograms. The zoo says the cubs are getting stronger by the day and are beginning the teething process.

The cubs are in the zoo's maternity den and not available for viewing by the public. 

The twin cubs were the first-ever giant pandas born in Canada. Their mom Er Shun is one of two pandas on loan from China.

All of the pandas will be at the Toronto Zoo until 2018, when they'll move to the Calgary Zoo for five years.