I don't know how to respond to jokes.

This lady at work had to sign for something on my handheld PC with this wand/stylus thingy.

It wasn't working but she finally signed it off and she finally said something like 'mmm, pencil's broken' with a grin.

I didn't know what to say to that. What do you normally say to little jokes like this?

Is there a stock standard reply?

Thanks.