How do you make your baby laugh?

My 4 month old laughed twice now. Once when I sang a song and made a funny sound (evanescence bring me to life lol) and once when I blew raspberries up her neck. But she got bored quickly and I am sooo desperate to hear that giggle again. So tell me - what do you do?

Edit: This thread makes me so happy! I tried a bunch - a few things scared her but the fake sneeze was a hit as well as speaking in a catholic church voice lol. But she needs a funny sound together with a feeling sensation to really giggle or squeak.