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Couldn't fit the whole question in the title. Can you defer admission to a PhD program you are accepted to, apply to other schools and, if you get into a better school go there and tell the school you deferred that you wont be coming?

My dilemma: I interviewed for a spot at a well known doctoral program, we'll call it school 1, and this morning I received an email to contact them via Skype for good news. I'm certain this is to tell me I'm accepted. I'm very excited to be accepted, and it's a great program! But... I have another interview coming up in February at school 2, which I'm also very excited about (probably a bit more excited than school 1) When I Skype the adviser at school 1, how can I communicate my excitement and enthusiasm, without accepting or turning down the offer? I need to keep school 1 in limbo, so to speak, for at least another month or so while I wait to hear back from other programs. Perhaps if this communication was via email I could write with enough finesse to accomplish this, but over Skype I'm trying to figure out how to navigate this. Help!