Stuck on Level 22 in AlphaBetty...how in the world do you pop a "blank" bubble???

The blank tiles are like the blanks in Scrabble, they can be any letter in the alphabet. So they are the most useful in my opinion.  The computer will automatically figure out any possible word combination using it.  So say you have the letters R A T and a blank tile.  You would select R - A -(blank)- T and it will eliminate all four tiles as the word RANT or RAFT they are helpful. If there is a word that can be made with the letter combinations the computer's dictionary will figure it out, you just need to put the blank in the right order with the rest of the letters to make a word.