Answer: A side drum
Suddenly the museum exhibits begin to move; a beat on a single drum, then three drums and a trumpet fanfare; swords are drawn and the ancient arms and armour spring into life. The invading Germans are met with an incredible sight confronted, as they are, by an army consisting of Vikings, armoured knights and redcoats.
"It's like the Lord Mayor's Show!" enthuses Charlie Rawlins as the magic army advances.
Battle is joined and the Germans, who stand no chance against their enchanted enemy, are beaten back into the sea... England is saved! Hurrah!
The film closes with Professor Browne marching off to war, promising to return to his new family - Eglantine, Charlie, Carrie and Paul - with the words "I'll be back before you've grown an inch taller; we'll be together again".