This is pretty common. It's simply the result of air escaping from the vagina. You might be familiar with the stages of arousal and orgasm. Just before orgasm, the vaginal walls balloon out a bit, sucking in air. When the vagina goes back to its normal state, the air escapes. Air might also get trapped in the vagina during certain sports.

Whatever the reason, it's not harmful, perhaps a little embarrassing, however. Making a joke about it might help to put you and your partner at ease.

There is nothing to prevent it entirely, but some say that using a bit of lube helps to prevent air from going into the vagina. You can also try what are called "Kegel exercises," which entail tightening the muscles that you use to stop the flow of urine. Practice several times a day when you go to the toilet or even when you're sitting on the bus--no one will know!

Good luck.