Either that or they have trampolines or pogo sticks.
The beer works well. I've used these traps before with lemonade too which works if you don't have beer in the house. I think any kind of sweet liquid will attract them.
A word of warning though, don't leave it too long before you empty it out and replace the beer. Drowned slugs are pretty disgusting anyway but even more so after a few days of fermentation! Empty it out each morning if there's anything in it.