"I went online to find out the differences between male and female pandas, and found that male pandas are more active, while female pandas are demure," says Chen, who spent eight long months on research.

But the most challenging part of the design was, he did not get to see the pandas for himself before starting work. Instead, he was given about a dozen photos of them.

"From the photos, I noticed that one of them had a roundish onion head with a straight black patch on the eye; and the other, a square mantou (bun) head with a curved black patch on the eye.

"Based on their photos, I guessed that Jia Jia was the one with the mantou head and curved black patch," he says.

Chen was spot on, as confirmed by the panda keepers later.

The design itself took about two intensive weeks, so intense that he fell ill right after completing the project.

"I rejected all other jobs during that period and locked myself up, staring at the panda photos and trying to grasp their spirit," he recalls.

"I had to draw every single fur. There was a lot of layering involved. For example, the white fur is not pure white but a combination of white and other colors. The problem with colored pencils is, if I make a mistake, I can't modify it. I have to start all over again."

Singapore Post approved his first and only draft, after a group of experts scrutinized his work.

This was unlike his first set of designs for the SingPost depicting otters. That revision took a depressing six months, which hampered his confidence when he started designing the panda stamps.

"The process and end result (of the panda stamps) were very satisfying," he adds.

Chen did not train as a graphic designer. Coming from a humble family background, he did not have many toys or gadgets as a child. That forced him to look for ways to amuse himself.