Start animation with a keystroke

I have a line on a client who wants someone to create an animation that “starts with a keystroke”. Info is a bit fuzzy right now, but I’m pretty sure that what he means to say is that he wants there to be an initial frame or scene and then for it to proceed, he just hits “ENTER” or whatever. I’ve never done anything like that before. Is it possible? And if so, can someone just give me a point in the right direction? This guy wants me on-site tomorrow, so any quick help would be much appreciated and bring glorious karma and stuff. Thanks in advance!