Another favorite trick of Houdini. He would invite the audience to examine a series of razor blades to verify they were real, put them in a glass of water, and then drink them. Moments later, he would slowly pull them out of the back of his throat, all connected together with string.
In reality, the blades he pulled from his mouth were coming from a packet stored between his cheek and his teeth. The razors the audience saw were surreptitiously spat back into the water instead of swallowed.