allow me to demonstrate using pictures and words.

step 1: get a bar plug (some old cinelli ones work great)

step 2: locate a friends beer

step 3: insert plug into beer

step 4: press down until it pops into place. man thats flush!

this is hard but not impossible to remove, making it a perfect prank

You forgot step 5: Shake beer.

Man, how come you had to demo with Kolsch? Haven't been able to get any here. :(

Didn't we already esablish that you live in a strange parallel universe or something?

Step 6: Friend strikes side of your face between eyebrow and cheekbone with bottle.

Step 7: Friend puts an extra spoke magnet on your wheel and then gives you riding directions with correct milage