Rudyjax said:
Here are 2 jokes that a modern audience who was not around in the 1980s wouldn't get.
Back to the Future:
Can I get a Pepsi Free?
Ok, how about a Tab?
This scene never made much since...
He first asks for a tab, which is sugar free and the precursor to diet coke (and coincidentally existed for nearly 40 years along side diet coke ). The waiter says 'I can't start a tab until you order something.'
Joke #1.
So Marty asks for a pepsi free, which is caffeine free pepsi. Not sugar free.
Waiter says you want a pepsi you have to pay for it. Joke #2.
Then Marty settles on 'something without any sugar in it', and waiter gives him coffee.
First, pepsi free is not sugar free, it's caffeine free. His final request of 'something without sugar' suggests he wouldn't want a pepsi free, but rather a diet pepsi free.
Second, what person would ask for a coke product by name, and then also a pepsi product.. you're usually either one or the other! Every 80s kid knows this