Some give their job offers on the final day of the internship and ask them to commit right away. The exploding offer is increasingly commonly given at the end of the summer, to both undergraduates and MBA students. There is usually no such guarantee, however, for summer interns. Most companies will give candidates some time to mull over the job offer. But other kinds of pressure may come into play.
Many companies offer a juicy signing bonus to candidates.
Under the exploding scheme, however, the amount of the signing bonus decreases the longer the candidate takes to make up her mind. That's some pretty costly indecision. You have a few options. First of all, you could opt to either accept or reject the offer immediately. If you know you want the job, then go ahead and take it.