Scratch

Try this:

when green flag clicked
set [yVelocity v] to (0)
forever
change y by (yVelocity)
if <touching color [ground]?>
set [yVelocity v] to (0)
if <key [space v] pressed?>
set [yVelocity v] to (10)
end
else
change [yVelocity v] by (-1)
end