Getting ketchup out of the bottle…

What you need:

A bottle of tomato ketchup (not the squeezy kind, that's cheating).
A drinking straw (the bendy kind).
A hotdog (the 'cooked through so it doesn't kill you' kind).

What you do:

Prepare the hotdog for ketchup addition.
Invert the ketchup bottle over the hotdog.
Note the lack of ketchup movement.
Note the continued lack of ketchup movement.
Turn the bottle upright again.
Stick your thumb over the end of the straw, then plunge it into the ketchup, ideally so it goes right to the bottom of the bottle.
Remove your thumb, and bend the straw over the neck of the bottle.
Hold the bottle neck and straw with one hand.
With a deft flick, invert the bottle and straw over the hotdog.
Observe the dramatic flow of ketchup.
Wonder if, on reflection, that wasn't a tad more ketchup than you wanted on your hotdog.

What's going on:

Ketchup doesn't come out of the bottle for a number of reasons, but fundamentally because it's very thick. For the ketchup to get out, air has to get in to fill the space it leaves behind, and the thick ketchup tends to plug the neck of the bottle. Plugged neck = no air in = no ketchup out. Stalemate.

The straw provides an alternative route for the air: it can get in simultaneously with the ketchup getting out. Free flow of air = free flow of ketchup = empty bottle in seconds flat. Try it, it's quite dramatic. You stick your thumb over the straw when you insert it so it stays full of air, rather than itself getting clogged with ketchup.

Exactly this idea is used when refuelling Formula 1 racing cars - the tanks can be filled much more quickly if air is pumped out while fuel is pumped in. Of course, the fuel should be high-octane petrol rather than ketchup. If it isn't, somebody's made an almighty mistake, and you shouldn't eat the pitlane hotdogs.