Easy...ask Paula...shes done 100% of things.
I used to do a similar "quick question" for pub nights...a sort of "on the hour" basis, for guesses up until about 5 minutes before the next hour (and question) was asked...the pub/punter retention level was superb...people would beg me for the answers at half past whatever, saying that they had to leave "now", but then took their coat off, bought another round (£££) and waited until the top of the hour.
I used to use a copy of "The book of top 10 lists of everything" a large A4 style book, which WHSmiths sell when its just out (every year) and all those discount bookstores seem to sell at a fraction of the price, 6 months later.
From its pages you could ask "in 2002, what was the top selling confectionary bar" (eg. Kit Kat)"
or "which country boasts the largest number of mobile phone handsets?" etc