I don't go to bars but have done this at other gatherings. Have three large glasses and two very small glasses. Fill two of the large glasses completely and the third one only about a fourth of the way. Fill the two very small glasses up and you are ready for the bet. Bet someone that you can drink the large glasses before they can drink the two very small glasses. You tell there is only two rules. The first is that no one can touch their glasses or disturb them in any way and no one can touch your glasses or disturb them in any way. If that happens you are disqualified and the other person is the winner. The second rule is that you get to drink the glass that is filled 1/4 of the way up first to clear your throat and they can't pick up their first glass until you sit down the first glass that you just drank. When they fully understand those two rules and agree to them. You say OK I'm going to drink my small one and when I set it down you can begin to drink your glasses. As you pick up your glass you can see the excitement on the other persons face because they know that they can drink those two very small glasses before you drink yours. Pick up your glass and sip at it a little just to tease them as if your are really trying to clear your throat and then drink the rest of that glass as they start to reach for their first very small glass you simply turn the glass that you just finished upside down over one of their very small glasses and say remember you cannot touch my glass or disturb it in any way or you loose. You can then take your time drinking the other two large glasses and win the bet. You will have fun with this as long as you aren't betting a sore headed person

Another common one, but it's a classic. I sometimes just say I want to "warm up" first

It's a good way to get some free bubble teas when people who don't drink