The possibilities are seemingly endless — and messy — for 7-Eleven’s “Bring Your Own Cup” day, in which customers can get their container filled with a Slurpee of their choice for only $1.50.

The promotion takes place on March 18 and March 19 in honor of the 50th anniversary of the frozen drink, but there are a number of rules to prevent stupidity, germs and general mayhem.

Just like it does for Donald Trump, size matters and in this case, customers will be penalized if what they’re holding is too “yuge” to fit in the slot.

When upright, the container must fit through a 10-inch circular hole.

And more importantly, the container must be “food safe,” so this clearly isn’t allowed:

shoe-gif-drinking

It also has to be watertight, further ruling out most shoes.

Finally, only one container per person is allowed.