iPad starts playing music in my bag!

i put my ipad in my bag and then after walking a bit, it starts playing music by itself. so obviously some jostling caused some combination of button hits on the ipad's outer controls caused music to autoplay.

can anyone tell me which buttons play music without opening up ipod in the ipad itself? sort of like me hitting the button on my iphone headset, when there is no active call, will start playing music on my iphone.

thx