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How to ask for offer deadline extension?

I received an offer for an internship but I still have 2 more office visits for other firms. The offer deadline is right before the office visits, so I need more time. I want to ask for an extension but am just wondering the best way to phrase it. I did like the firm that gave me an offer, but want to see if I will receive any additional offers from the remaining 2. Do I just say "I'm visiting other firms" or would that be the wrong thing to do? Please tell me what is generally said when asking for an extension.

I definitely don't want to accept then end up renegging. I am pretty sure the firm will grant an extension, I just want to know the best way to put my request in an email? If they happen not to grant the extension, I probably would accept and back out if I really felt like it was the best thing to do.