Split the class into two groups and form 

a single file line down the left hand side and the right hand side of the room. Put the inflatable hammer on the desk in front of the class.

The first student from each group and stand facing each other in front of the teacher, where the hammer is.

One student says an English word (any word is OK, for example "banana") and th

e other student responds by saying another English word. Then the two students go to the back of the line. The second students in line repeat the process.

The game continues until one student repeats a word (i.e. the student says a word that another pair have already said).  The person who say a word that has been said, gets a hit on the head or shoulder unless s/he puts on the hat before the winner hits him/her.  Then this student sits down and the other person, the winner, rejoins his/her group.

Similarly if students speak in Japanese, pause or hesitate, then they get a bash on the head unless they put on the hat and have to sit down, but their opponent rejoins the group.

When the group runs out of members, the group loses and the other group wins.