Paperclip Catapult

This is low cost and you only need a paperclip and some tape (i use masking tape)

Step 1:

Bend up the small end of the paperclip

Step 2:

Wrap sellotape round the small end of the paperclip

Step 3:

Gat the blue tack and make a tiny ball

Step 4:

Put blue tack on catapult and fire